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It is a crucial challenge to address both magnetic anisotropy and stability for single-molecule magnets (SMMs) used in next-generation nanodevices. Highly axial lanthanide SMMs with neutral charge ...

Double-hybrid (DH) approximations have entered into the limelight of density functional theory (DFT) computations of different properties; however, little is known regarding their accountability for spin-state energetics in transition metal complexes. In this work, taking high-level all-electron fixed-node diffusion Monte Carlo data as a reference, we present a survey of the applicability of parameterized and parameter-free DHs as well as their dispersion and non-local corrected versions for predicting the spin splitting energies of transition metal complexes collected from the literature and from our own proposals herein. Our proposed parameter-free DH based on the spin-opposite-scaled (SOS) scheme incorporating the Perdew-Burke-Ernzerhof (PBE) exchange and strongly constrained and appropriately normed (SCAN) correlation as well as high balanced fractions of nonlocal exchange and correlation without any additional correction, SOS0-PBESCAN0-2(a), is found to be superior for overall performance. This model not only surpasses other DFT approximations from various rungs of the "Jacob's Ladder" classification and recently reported DHs for the present purpose but also outperforms wave function-based approaches in most cases. Dissecting the roles played by the non-local exchange and correlation contributions as well as their interplay, it is shown that this good performance arises mainly from an appropriate compromise between energy- and density-driven errors. Furthermore, by employing the proposed model and a variety of modified versions thereof, we scrutinize the roles of various factors, such as the ligand field strength and oxidation state of the metal ions, in both qualitative and quantitative descriptions of spin-state energetics in other complexes with different metals and ligands.

We applied density functional theory calculations to ruthenium–nitrosyl complexes, which are known to exist in high-level radioactive waste generating during reprocessing of spent nuclear fuel, to give a theoretical correlation between 99Ru Mossbauer spectroscopic parameters and ligand field strength for the first time. The structures of the series of complexes, [Ru(NO)L5] (L = Br–, Cl–, NH3, CN–), were modeled based on the corresponding single-crystal X-ray coordinates. The comparisons of the geometries and total energies between the different spin states suggested that the singlet spin state of [Ru(II)(NO+)L5] complexes were the most stable. This result was supported by the benchmark calculations of the 99Ru Mossbauer isomer shift (δ) and quadrupole splitting (ΔEQ) values. The calculated results of both the δ and ΔEQ values reproduced the experimental results by reported previously and increased in the order of L = Br–, Cl–, NH3, CN–. Finally, we estimated the ligand field strength (Δo) based on molecul...

For the purpose of further exploring the effect of nonperipherally attached substituents on single-ion magnet (SIMs) performance, tetrasubstituted bis[1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninato]terbium double-deckers, in both the reduced form TbH[Pc(α-OC5H11)4]2 (1) and the neutral form Tb[Pc(α-OC5H11)4]2 (2), were prepared. Single-crystal X-ray diffraction analysis for 2 unambiguously demonstrates the pinwheellike molecular structure with C4 symmetry. Magnetic investigations of the two bis(phthalocyaninato)terbium double-deckers reveal their characteristic SIM nature. 2 exhibits SIM performance superior to that of 1, as revealed by the larger energy barrier of 466 K for the former species and 431 K for the latter species due to the presence of organic radical–f (radical–Tb) interactions. The enhanced SIM performance of 2 in comparison to 1 actually stems from the presence of radical–f interactions and an enhanced ligand field strength. The latter positive factor is indicated by the electrostatic pot...

The pentafluorophenylethynyl ligand, C2C6F5, is a commonly used electron withdrawing ligand. In the context of developing complexes with blue-shifted emission for organic light emitting diode applications, it is often necessary to have electron poor ligands that are also relatively high on the spectrochemical series. The trifluoropropynyl ligand, C2CF3, is also electron withdrawing and has never been directly compared to C2C6F5. Herein complexes of the type trans-[M(cyclam)(C2R)2]OTf, where M = CoIII and CrIII and C2R = C2C6F5, are prepared and characterized by X-ray crystallography, cyclic voltammetry, and UV-Vis, IR, and Raman spectroscopy. Emission spectroscopy for the CrIII complex is also reported. These C2C6F5 complexes are compared to the corresponding complexes where C2R = C2CF3, and C2C6H5. For trans-[Co(cyclam)(C2R)2]OTf, the reversible CoIII/II reduction for the C2C6F5 complex occurs 120 mV more cathodic than that for the C2CF3 complex, indicating that C2CF3 is more electron withdrawing, a conclusion that is supported by analysis of C C stretching frequencies. The lowest energy metal-centered (MC) 1A1g → 1T2g absorption band for the C2CF3 and C2C6F5 complexes in CH3CN appear at 425 and 438 nm, respectively, suggesting that C2CF3 is a stronger field ligand. In the region of the LMCT transitions, the absorption spectra for the trans-[Cr(cyclam)(C2R)2]OTf complexes, where C2R = C2C6H5 and C2C6F5, show significant vibronic structure that is also superimposed on the MC transitions, suggesting some degree of charge transfer character in the latter. The CrIII complexes are all emissive in room-temperature fluid solution with the C2CF3 complex showing significant fine structure and the C2C6H5 and C2C6F5 complex emission being broad, structureless, and redshifted by comparison. This suggests that emission in the C2CF3 complex is of 2Eg (Oh) origin and emission for the C2C6H5 and C2C6F5 complexes is of 2T1g (Oh) origin. Emission for all CrIII complexes in 77 K glass shows significant fine structure. The presence of a dominant 0-0′ emission band in the C2C6H5 and C2C6F5 complexes suggests a non-centrosymmetric ground state. This is supported by the crystal structure for trans-[Cr(cyclam)(C2C6F5)2]OTf that shows an angle of 77.8° between the planes defined by the aryl rings.

Self-emitting blue and red EuOX (X = F, Cl, Br, and I) were successfully synthesized and characterized. Far-infrared and Raman measurements revealed that the vibration modes prominently reflected the Eu-O and Eu-X bond characters of these materials. X-ray photoelectron spectroscopy (XPS) of the red-emitting EuOX compounds showed that Eu exclusively existed as Eu3+, while in the blue-emitting EuOX, a mixed Eu3+/Eu2+ state was observed. For the red-emitting EuOX (X = F, Cl, and Br), the maximum wavelengths of the charge-transfer (CT) bands were red-shifted: F → Cl → Br (282, 320, and 330 nm for F, Cl, and Br, respectively). Using one-electron spin-polarized band structure calculations, it was verified that the red-shift of the CT energy from F to Br in EuOX was mainly due to the relative positions of the halogen orbital energies being gradually increased, following the trend in their electronegativity. For the blue-emitting EuOX (X = Cl, Br, and I), the emission band maxima were red-shifted from Cl to I (409, 414, and 432 nm for Cl, Br, and I, respectively), which was quite opposite to the trend predicted based on the spectrochemical series in crystal field theory, which was in good agreement with the previous results of the calculated 5d → 4f transition energies of the Eu2+ activator based on the crystal field theory. Through photoluminescence, UV-visible absorbance, and XPS, it was elucidated that the red emission due to Eu3+ was strongly masked by the intensified blue emission associated with the small amount of Eu2+ in the blue-emitting EuOX (X = Cl, Br, and I). These materials may provide a platform for modeling new phosphors for application in solid-state lighting.

The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–

The quintet-singlet energy difference (Δ EQ/S) in Fe(II) polypyridine complexes is often interpreted in terms of metal-ligand π interactions. DFT calculations on a series of substituted [Fe(bpy)3]2+ (bpy = 2,2'-bipyridine) complexes show the disparate magnitudes of substituent effects on tuning Δ EQ/S and reduction potentials ( E°). In this series, E° spans a much larger range than Δ EQ/S (2.07 vs 0.29 eV). While small changes in Δ EQ/S are controlled by metal-ligand π interactions, large changes in E° arise from modification of the electrostatic environment around the Fe center. Molecular orbital analysis reveals that, contrary to the typical description of bpy as a π-acceptor, bpy is better described as acting as both a π-donor and π-acceptor in [Fe(bpy)3]2+ complexes, even when it is substituted with highly electron withdrawing substituents. Overall, substituent modification is a useful strategy for fine-tuning the ligand field strength but not for significant reordering of the spin-state manifold, despite the large effect on metal-ligand electrostatic interactions.

Homoleptic complexes [Fe(Ln)]X2 (L1 = 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, L2 = 1,1′-(α,α′-3,4-dibromo-o-xylyl)-2,2′-biimidazole, L3 = 1,1′-(α,α′-2,5-dimethoxy-o-xylyl)-2,2′-biimidazole; X = BF4– or ClO4–) were synthesized by direct reactions of the Fe(II) precursor salts and bidentate ligands L1, L2, or L3. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Complexes with ligands L1 and L2 synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L3, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe(L3)3](BF4)2 and ∼1:1 for [Fe(L3)3](ClO4)2. Complexes with L1 can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L3. The differences in magnetic behavior ...

The ligand field splitting Δt in a number of homoleptic tetrahedral complexes of Cu(I) and Ni(0) are determined from electronic structure calculations by DFT at the BP86/6-31+G(d,p) and BP86/LanL2DZ levels. Some ligands are monodentate (e.g. CO, NH3, Cl− etc.) and some are bidentate (e.g. ethylenediamine, 2,2′-bipyridine etc.). The Δt values obtained with the two basis sets for a given metal ion are comparable except for the phosphines. They correlate linearly with Shimura’s dCo parameters which are considered to be most effective for constructing a spectrochemical series. Earlier it was believed that the dCo parameters are applicable only to the octahedral complexes of cobalt(III). It is further shown that the dCo parameters bear a linear relationship with Chatt’s ligand constants PL. Implications are discussed.

In this work, a general, user-friendly method – ab initio ligand field theory (AILFT), is described and illustrated. AILFT allows one to unambiguously extract all ligand field parameters (the ligand field one-electron matrix V LFT , the Racah parameters B and C , and the spin-orbit coupling parameter ζ ) from relatively straightforward multi-reference ab initio calculations. The method applies to mononuclear complexes in d n or f n configurations. The method is illustrated using complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) calculations on a series of well documented octahedral complexes of Cr III with simple ligands such as F − , Cl − , Br − , I − , NH 3 and CN − . It is shown that all well-known trends for the value of 10 Dq (the spectrochemical series) are faithfully reproduced by AILFT. By comparison of B and ζ for Cr III in these complexes with the parameters calculated for the free ion Cr 3+ , the covalency of the Cr-ligand bond can be assessed quantitatively (the non-relativistic and relativistic nephelauxetic effects). The variation of ligand field parameters for complexes of 3d, 4d and 5d elements is studied using MCl 6 3− (M = Cr III , Mo III , W III ) as model examples. As reflected in variations of 10 Dq , B and ζ across this series, metal-ligand covalency increases from CrCl 6 3− to MoCl 6 3− to WCl 6 3− . Using the angular overlap model, the one-electron parameters of the ligand field matrix are decomposed into increments for σ- and π- metal-ligand interactions. This allows for the quantification of variations in σ- and π-ligand donor properties of these ligands. Using these results, the well documented two-dimensional spectroscopic series for complexes of Cr III is quantitatively reproduced. Comparison of the results obtained using CASSCF and NEVPT2 reveals the importance of dynamic electron correlation. Finally, the limitations of the AILFT method for complexes with increasing metal-ligand covalency are analyzed and discussed.

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal–organic frameworks (MOFs) of the following formula: [(CH3)2NH2][K0.5Cr0.5(HCOO)3] (DMAKCr) and [C2H5NH3][K0.5Cr0.5(HCOO)3] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R space group) and undergoes an order–disorder phase transition to the monoclinic system (P space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal–formate framework. EtAKCr crystallizes in a monoclinic structure (P21/n space group) with ordered EtA+ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na+ ions by larger K+ ions in the [cat]MIMIII (cat = DMA+, EtA+, MI = Na+, K+ and MIII = Cr3+ and Fe3+) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal–formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr3+-based emission characteristics for intermediate ligand field strength.

Temperature-dependent optical absorption spectra have been recorded for Cr3+ doped silicate glasses between 10 K and 773 K. Due to the intermediate ligand field strength of the Cr3+ coordination environment, overlap of the lowest-energy spin-allowed 4A2 → 4T2 band and the two spin-forbidden transitions to the 2E and 2T1 states at approximately 650 nm results in interference dips. An analytical model equation is fitted to the spectra and electronic structure parameters extracted. This represents the first time that variations of these parameters with temperature are obtained over a large temperature range, revealing characteristics of all three overlapping d-d transitions. Keywords: Chromium(III) doped glasses, Absorption spectra, Interference dips, Temperature, Spectra calculations

The original magnetism of a model spin crossover complex [Fe(H2Bpz2)2(bipy)] (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy = 2,2′-bipyridine) has been altered from spin state switching to paramagnetic behavior, through grafting a weak electronic-donating group: bromomethyl (in 1) and Br (in 2) onto the bipy ligand at positions C5 and C5´. The introduction of the electron-donating bromo groups into the bipy ligand reduce the π-acceptor character, leading to a weaker ligand field strength, thus stabilizing the high-spin state.

Two unique trinuclear terpyridine-ruthenium complexes having similar components placed at different angels were successfully synthesized and fully characterized for the purpose of determining the photophysical and electronic properties. The corresponding UV–vis absorptions and similar electrochemical properties showed characteristic MLCT transitions in both complexes. However, complex C2 displayed an intense emission at 77 K, as expected, where highly packed complex C1 is almost non-emissive. This result may due to that a distinct configuration adopted by the complex impacts ligand field strength.

Two iso-structural two-dimensional (2D) porous coordination polymers, which formulated as [Fe(1,3-di(4-pyridyl)propane)2(NCX)2]2 (X = S [1], Se [2]) and featured 2-fold interpenetration, have been synthesised and characterized. Crystal structure, Hirshfeld surfaces analysis and magnetic properties of them were described. The basic-unit of 1 and 2 was the 2D rhombic grids [2 × (4, 4)] network, and the intermolecular interactions in the crystals were dominated by C–H…π, H–H and C–H…S (Se) contacts. The stronger ligand field strength of SeCN− than that of SCN− resulted to different magnetic properties: compound 1 shows paramagnetic property while 2 display displays an incomplete SCO transition approximately 30% conversion of high-spin (HS) to low-spin iron(II) sites with transition temperature of 100 K.

The crystal field theory as applied to complex compounds of d-elements surrounded by octahedral ligans was described. Ligand field causes the splitting of d-orbitals, which is characterized by the energy splitting Δo. The spectrochemical series of ligands and examples of high-spin and low-spin complex compounds depending on the degree of force field were presented. Deformation of octahedral complexes by the Jahn-Teller effect was described. It shows the calculation of gains power as a result of complex formation, called the crystal field stabilization energy (CFSE) depending on the electronic structure of the central ion and the ligand position in spectrochemical series. It shows the distribution of electrons in orbitals of eg and t2g complex ions with different electronic configurations (from d1 to d10), examples values of energy orbitals and energy of crystal field stabilization for high-spin and low-spin octahedral complexes. Keywords: crystal field, ligand, splitting energy, сentral atom, d-orbital, speсtrochemical series, tetrahedral complex, octahedral complex, low-spin complexes, high-spin complexes.

The resulting distortion of the octahedral symmetry of the complex [CrIII(NH3)6]3+ upon replacing the axial ligands with halides (i.e., weaker ligands) affects the stability of the doublet state with respect to that of the quartet ground state. This substitution affects the doublet-to-quartet transition responsible for phosphorescence. The position of the halide with respect to ammonia in the spectrochemical series is a major influence on the emission wavelength of the complex. The close proximity of fluorine and ammonia in the spectrochemical series leads to a blueshift in the emission wavelength when fluoride ions are introduced into the complex, thus providing a rational approach to the design of blue-phosphorescent materials, which are desirable for OLEDs used in full-color displays.

Copyright © 2018 American Chemical Society. Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV, whereas the polaron formation probability (85 ± 10%) remains constant. By comparison to hematite (α-Fe2O3), an oxide analogue, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.

International audience; Luminescence spectra of isoelectronic square-planar d8 complexes with 3d, 4d, and 5d metal centers show d–d luminescence with an energetic order different from that of the spectrochemical series, indicating that additional structural effects, such as different ligand–metal–ligand angles, are important factors. Variable-pressure luminescence spectra of square-planar nickel(II), palladium(II), and platinum(II) complexes with dimethyldithiocarbamate ({CH3}2DTC) ligands and their deuterated analogues show unexpected variations of the shifts of their maxima. High-resolution crystal structures and crystal structures at variable pressure for [Pt{(CH3)2DTC}2] indicate that intermolecular M···H–C interactions are at the origin of these different shifts.

Over the past several decades, tremendous efforts have been invested in finding molecules that display slow relaxation of magnetization and hence act as single-molecule magnets (SMMs). While initial research was strongly focused on polynuclear transition metal complexes, it has become increasingly evident that SMM behavior can also be displayed in relatively simple mononuclear transition metal complexes. One of the first examples of a mononuclear SMM that shows a slow relaxation of the magnetization in the absence of an external magnetic field is the cobalt(II) tetra-thiolate [Co(SPh)4](2-). Fascinatingly, substitution of the donor ligand atom by oxygen or selenium dramatically changes zero-field splitting (ZFS) and relaxation time. Clearly, these large variations call for an in-depth electronic structure investigation in order to develop a qualitative understanding of the observed phenomena. In this work, we present a systematic theoretical study of a whole series of complexes (PPh4)2[Co(XPh)4] (X = O, S, Se) using multireference ab initio methods. To this end, we employ the recently proposed ab initio ligand field theory, which allows us to translate the ab initio results into the framework of ligand field theory. Magneto-structural correlations are then developed that take into account the nature of metal-ligand covalent bonding, ligand spin-orbit coupling, and geometric distortions away from pure tetrahedral symmetry. The absolute value of zero-field splitting increases when the ligand field strength decreases across the series from O to Te. The zero-field splitting of the ground state of the hypothetical [Co(TePh)4](2-) complex is computed to be about twice as large as for the well-known (PPh4)2[Co(SPh)4] compound. It is shown that due to the π-anisotropy of the ligand donor atoms (S, Se) magneto-structural correlations in [Co(OPh)4](2-) complex differ from [Co(S/SePh)4](2-). In the case of almost isotropic OPh ligand, only variations in the first coordination sphere affect magnetic properties, but in the case of S/SePh ligand, variations in the first and second coordination sphere become equally important for magnetic properties.

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm⁻¹, E = 0.1(2) cm⁻¹ and D = 13.4(6) cm⁻¹, E = 0.3(6) cm⁻¹ for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the ⁶A₁ ground state. D was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆³⁻ complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π- and σ-antibonding energies e(λ)(X) (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. D parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental D and spectroscopic transitions for high-spin Fe(III) complexes are proposed.

Reaction of silver(I) with robust halido complexes of Ir(III) leads to second sphere coordination rather than halide abstraction. Two hexa-coordinated pseudo-octahedral silver(I) complexes [(CH 3 CN) 3 IrX 3 AgX 3 Ir(NCCH 3 ) 3 ] + (X = Cl, Br) have been synthesized and structurally characterized as PF 6 − and SbF 6 − salts, respectively. The synthesis and crystal structure of the new, mononuclear complex, [IrBr 3 (NCCH 3 ) 3 ], are also reported along with spectroscopic characterization of [IrX 3 (NCCH 3 ) 3 ] and the trinuclear silver complexes. The electronic structures are analyzed in the framework of ligand field theory and by comparison with high-level correlated multireference ab initio calculations. The ligand field strength of the heavier halide complexes is found to be significantly less sensitive to outer-sphere complexation as compared to the pronounced outer sphere ligation effects commonly observed for fluoride complexes.

Fe(II) polypyridines are an important class of pseudo-octahedral metal complexes known for their potential applications in molecular electronic switches, data storage and display devices, sensors, and dye-sensitized solar cells. Fe(II) polypyridines have a d(6) electronic configuration and pseudo-octahedral geometry and can therefore possess either a high-spin (quintet) or a low-spin (singlet) ground state. In this study, we investigate a series of complexes based on [Fe(tpy)2](2+) (tpy = 2,2';6',2″-terpyridine) and [Fe(dcpp)2](2+) (dcpp = 2,6-bis(2-carboxypyridyl)pyridine). The ligand field strength in these complexes is systematically tuned by replacing the central pyridine with five-membered (N-heterocyclic carbene, pyrrole, furan) or six-membered (aryl, thiazine-1,1-dioxide, 4-pyrone) moieties. To determine the impact of ligand substitutions on the relative energies of metal-centered states, the singlet, triplet, and quintet states of the Fe(II) complexes were optimized in water (PCM) using density functional theory at the B3LYP+D2 level with 6-311G* (nonmetals) and SDD (Fe) basis sets. It was found that the dcpp ligand scaffold allows for a more ideal octahedral coordination environment in comparison to the tpy ligand scaffold. The presence of six-membered central rings also allows for a more ideally octahedral coordination environment relative to five-membered central rings, regardless of the ligand scaffold. We find that the ligand field strength in the Fe(II) polypyridines can be tuned by altering the donor atom identity, with C donor atoms providing the strongest ligand field.

Nickel(II) complex with sarcosine [Ni(sar)2(H2O)2] (1) has been obtained as a reaction product of a system: Ni(II)Cl2–1-methylhydantoin. Basic hydrolysis of the organic substrate leads to in situ formation of sarcosine ligand, which coordinates to the metal ion. The isolated complex has been characterized by means of single crystal X-ray diffraction and spectroscopic studies (IR, Raman, and NIR–UV–Vis) supported with DFT calculations. Single crystal X-ray diffraction revealed that the nickel environment in [NiN2O4] chromophore exhibited the geometry of a tetragonally elongated octahedron. Hydrogen bonds create two chain patterns which propagate along the main crystallographic directions a and b. It leads to the formation of a 2D network. Electronic spectra analysis showed that the nickel surroundings can be described as pseudooctahedral in solution and tetragonal in the solid state. Based on the calculated 10Dq parameter (10,990 cm−1), sarcosine ligand was located in the spectrochemical series close to ammonia. Comprehensive studies of the molecular structure and vibrational spectra of the title complex have been performed using UPBE0, unrestricted density functional method. The clear-cut assignment of the bands in FT-IR and Raman spectra of studied complex has been made on the basis of the calculated potential energy distribution. The Ni–L(sarcosine) stretching vibrations were assigned in Raman spectrum to the medium intensity band at 442 cm−1.

The fundamental balance between high- and low-spin states of transition metal systems depends on both the metal ion and the ligands surrounding it, as often visualized by the spectrochemical series. Most density functionals do not reproduce this balance, and real spin state propensities depend on orbital pairing and vibrational entropies absent in the spectrochemical series. Thus, we systematically computed the tendency toward high or low spin of "text-book" octahedral metal complexes versus ligand and metal type, using eight density functionals. Dispersion effects were generally5 kJ/mol, favoring low-spin states. Zero-point energies favored high-spin states up to 33 kJ/mol for strong ligands, but down to a few kilojoules per mole for weak ligands. Vibrational entropy also favored high-spin states up to 40 kJ/mol, most for strong ligands. Jahn-Teller distortion in Co(II) low-spin states, particularly stable d(6) low-spin states, and entropy corrections were consistent with experiment. Entropy and zero-point energy corrections were markedly lower for Co(II) and Mn(III), viz., the differential ligand field stabilization energy, and can only be ignored for weak ligands. The data enable simple assessment of spin state propensities versus ligand and metal type and reveal, e.g., that CN(-) is consistently weaker than CO for M(II) but stronger than CO for M(III) and SCN(-) and NCS(-) change order in M(II) versus M(III) complexes. Contrary to expectation based on the spectrochemical series, Cl(-) and Br(-) are very close in spin state propensity because the pairing penalty for low spin is smaller in Br(-). Thus, for the M(II) complexes, we find a consensus order of Br(-) ∼ Cl(-)H2OSCN(-)NCS(-) ∼ NH3CN(-)CO, whereas for the M(III) complexes, an approximate order is Br(-) ∼ Cl(-)H2O ∼ NCS(-) ∼ SCN(-)NH3COCN(-).

A new spin crossover coordination polymer (SCO-CPs) of Fe(II)-4,4′-bipyridine (4,4′-bipy) family: {Fe(4,4′-bipy)2(H2O)2}·(4,4′-bipy)· 8(H2O)·2(ClO4) (3), which displays half spin transitions between 100 and 300 K, has been synthesized and structurally characterized. Compound 3 featured with two-dimensional (2-D) grids connected by hydrogen bonds and π…π packing between one-dimensional (1-D) chains, the 2-D grids expand to three-dimensional (3-D) architecture supported by a “S-shaped holder” involving lattice 4-4′-bipy, water molecules and perchlorate anion. We compared 3 with the other two analogous complexes: ({Fe(4,4′-bipy) (H2O)2 (NCS)2}·4,4′-bipy, 1 and {Fe(4,4′-bipy)2(NCS)2}·mSolv, 2) through Hirshfeld surfaces analysis, which revealed that the low ligand field strength (NCS−) and lone-pair…H contacts contribute to the stabilization of HS (high-spin) state of the Fe(II) ion, while the high ligand field strength (4,4′-bipy) and strong intermolecular contacts (hydrogen bonds and π…π packing interactions) make for the LS (low-spin) state.

This paper explores the strengths and limitations of valence-to-core X-ray emission spectroscopy (V2C XES) as a probe of coordination environments. A library was assembled from spectra obtained for 12 diverse Cr complexes and used to calibrate density functional theory (DFT) calculations of V2C XES band energies. A functional dependence study was undertaken to benchmark predictive accuracy. All 7 functionals tested reproduce experimental V2C XES energies with an accuracy of 0.5 eV. Experimentally calibrated, DFT calculated V2C XES spectra of 90 Cr compounds were used to produce a quantitative spectrochemical series showing the V2C XES band energy ranges for ligands comprising 18 distinct classes. Substantial overlaps are detected in these ranges, which complicates the use of V2C XES to identify ligands in the coordination spheres of unknown Cr compounds. The ligand-dependent origins of V2C intensity are explored for a homologous series of [CrIII(NH3)5X]2+ (X = F, Cl, Br, and I) to rationalize the variable intensity contributions of these ligand classes., Experimental data and density functional theory calculations that assess the ability of valence-to-core X-ray emission spectroscopy (V2C XES) to distinguish ligands coordinated to chromium are presented. Discussion extends to the factors dictating the intensity of features corresponding to ligand valence ns → metal 1s transitions.

In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated CoII complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of CoII centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO–SOMO) excitation contributes almost entir...

The spin-state orderings in nine Fe(II) and Fe(III) complexes with ligands of diverse ligand-field strength were investigated with multiconfiguration pair-density functional theory (MC-PDFT). The performance of this method was compared to that of complete active space second-order perturbation theory (CASPT2) and Kohn–Sham density functional theory. We also investigated the dependence of CASPT2 and MC-PDFT results on the size of the active-space. MC-PDFT reproduces the CASPT2 spin-state ordering, the dependence on the ligand field strength, and the dependence on active space at a computational cost that is significantly reduced as compared to CASPT2.

The effect of the phenolate ring derivatisation on the magnetic properties of Fe(III) complexes bearing Nethyl- N-(2-aminoethyl) salicylaldiminate ligands both in solid state and solution have been investigated. Two new complexes [Fe(3,5-Br-salEen)(2)]ClO4 center dot EtOH (5) and [Fe(3,5-Br-salEen)(2)]BPh4 center dot DMF (6) have been synthesised. SQUID magnetometry studies on these complexes showed that while complex 5 is in the low-spin (LS) state, complex 6 displays a gradual and incomplete spin crossover (SCO) transition over the temperature measured. Solution measurements on a series of six complexes e [Fe(salEen)2]ClO4 (1), [Fe(salEen)(2)]BPh4 center dot 0.5H(2)O (2), [Fe(5-Br-salEen)(2)]ClO4 (3), [Fe(5-Br-salEen)(2)]BPh4 center dot DMF (4), [Fe(3,5-Br-salEen)(2)]ClO4 center dot EtOH (5) and [Fe(3,5-Br-salEen)(2)]BPh4 center dot DMF (6) - were performed by UVevis and NMR spectroscopies and cyclic voltammetry. Solution studies show that the presence of electron withdrawing groups (bromine atoms) affect the electronic density at the phenolate ring, thus influencing the ligand field strength and the separation between the t(2g) and e(g)* energy levels. The presence of two bromide substituents at the phenolate ring has a more pronounced effect on the magnetic behaviour in solution than in the solid state, with both complexes 5 and 6 adopting preferentially the LS state. Electrochemical studies of complexes 1-6 reveal that the reduction of the metallic centres in the complexes with electron withdrawing groups is easier, with E-1/2 values of iron moving to more positive potentials with the number of bromide substituents at the phenolate ring. (C) 2013 Elsevier B. V. All rights reserved.

The present report describes the syntheses, structures and luminescence properties of five mononuclear complexes, [Zn(L)Cl]ClO 4 ( 1 ), [Zn(L)Br]ClO 4 ( 2 ), [Zn(L)I]ClO 4 ( 3 ), [Zn(L)N 3 ]ClO 4 ( 4 ), and [Zn(L)(NCS)](SCN) ( 5 ), derived from the tetradentate ligand L, N , N ′-bis(2-pyridylmethylene)-1,3-propanediamine. The geometry around the Zn 2+ centre is best described as square pyramidal, although significant distortion towards the trigonal pyramidal geometry is noticed, especially for the halide complexes, of the later, the chloro ( 1 ) and bromo ( 2 ) complexes are isostructural and isomorphous. The stabilization of the crystal lattices is maintained by interesting, relatively strong hydrogen bonds and π⋯π interactions which lead to various supramolecular architectures. All the five complexes show photoluminescence properties and their trend can be correlated with the position of the apical anionic coligands in the spectrochemical series of ligands.

New topologically constrained tetraaza macropolycycles L9, L10, and L11 containing two o-xylylene (–(CH2)C6H4(CH2)–) bridges have been prepared by the reaction of 1,2-bis(bromomethyl)benzene with 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07.12]docosane (L1), 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L2), or 2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L3). The macropolycycle L9 readily reacts with Cu2+ ion to form [CuL9]2+ in wet methanol. However, L10 and L11 do not bind the metal ion in similar conditions. The Cu–N distances [2.048(4)–2.070(4)] and the ligand field strength of [CuL9](ClO4)2 are considerably longer and weaker, respectively, than those of other related macropolycyclic copper(II) complexes containing two N-(CH2)2-N or N-(CH2)3-N bridges. The copper(II) complex is extremely stable even in low pH and quite inert against ligand substitution with CN− ion.

Specific ion binding by carboxylates (−COO–) is a broadly important topic because −COO– is one of the most common functional groups coordinated to metal ions in metalloproteins and synthetic polymers. We apply quantum chemical methods and the quasi-chemical free-energy theory to investigate how variations in the number of −COO– ligands in a binding site determine ion-binding preferences. We study a series of monovalent (Li+, Na+, K+, Cs+) and divalent (Zn2+, Ca2+) ions relevant to experimental work on ion channels and ionomers. Of two competing hypotheses, our results support the ligand field strength hypothesis and follow the reverse Hofmeister series for ion solvation and ion transfer from aqueous solution to binding sites with the preferred number of ligands. New insight arises from the finding that ion-binding sequences can be manipulated and even reversed just by constraining the number of carboxylate ligands in the binding sites. Our results help clarify the discrepancy in ion association between mo...

Crystal structures and magnetic properties of polymeric and trinuclear heterobimetallic MnIII···PtII···MnIII coordination compounds, prepared from the Ba[Pt(CN)4] and [Mn(L4A/B)(Cl)] (1a/b) precursor complexes, are reported. The polymeric complex [{Mn(L4A)}2{μ4-Pt(CN)4}]n (2a), where H2L4A = N,N’-ethylene-bis(salicylideneiminate), comprises the {Mn(L4A)} moieties covalently connected through the [Pt(CN)4]2− bridges, thus forming a square-grid polymeric structure with the hexacoordinate MnIII atoms. The trinuclear complex [{Mn(L4B)}2{μ-Pt(CN)4}] (2b), where H2L4B = N,N’-benzene-bis(4-aminodiethylene-salicylideneiminate), consists of two [{Mn(L4B)} moieties, involving pentacoordinate MnIII atoms, bridged through the tetracyanidoplatinate (II) bridges to which they are coordinated in a trans fashion. Both complexes possess uniaxial type of magnetic anisotropy, with D (the axial parameter of zero-field splitting) = −3.7(1) in 2a and −2.2(1) cm−1 in 2b. Furthermore, the parameters of magnetic anisotropy 2a and 2b were also thoroughly studied by theoretical complete active space self-consistent field (CASSCF) methods, which revealed that the former is much more sensitive to the ligand field strength of the axial ligands.

We report preparation and characterization of a novel complex cis–mer [Co(L)2]I2 CH3OH (1) (L-N,N-bis(1-(3,5-dimethylpyrazolylmethyl)amine)) isolated from the [Co0–1-hydroxymethyl-3,5-dimethylpyrazole–MoO3–NH4I] system. A two-ring scorpionate ligand (L) was obtained in situ via redox and condensation processes. The catalytical properties of MoO3 are responsible for the formation of this atypical scorpionate ligand. Compound (1) crystallizes in the orthorhombic space group Pna21 with a = 24.5578(14) A, b = 13.8388(8) A, c = 9.6072(7) A, V = 3265.0(4) A3 and Z = 4. The metal ion coordinates through six nitrogen atoms which belong to two tridentate ligands forming a deformed octahedron. The complex was characterized by IR, FIR, UV–VIS and magnetic investigations. The ligand was ranked in the spectrochemical series between imidazole and bipyridine. The crystal field splitting of tridentate N-donor L was found to be smaller in comparison to e.g. bidentate bipyridine.

Remarkably little is known about metal-to-ligand charge transfer (MLCT) excited-state relaxation pathways for the ruthenium polypyridyl compounds commonly utilized in dye-sensitized solar cells. Herein, we report variable-temperature photoluminescence studies of compounds of the general type cis-Ru(LL)2(X)2, where LL is a bipyridyl ligand and X is CN(-) or NCS(-), and contrast them with the well-known Ru(bpy)3(2+) and Os(bpy)3(2+), where bpy is 2,2'-bipyridine, to identify relaxation pathways. In fluid acetonitrile and propylene carbonate solutions, excited-state relaxation was found to obey a first-order kinetic model. An Arrhenius analysis revealed internal conversion to two different states, assigned to an upper MLCT excited state and a ligand field excited state. Relaxation through the upper MLCT excited state typically displayed pre-exponential factors of 10(7)-10(8) s(-1) with activation energies of 400-900 cm(-1), while relaxation rates through ligand field states occurred with 10(14)-10(15) s(-1) and activation energies of 4000-5000 cm(-1). Nonradiative decay through LF states was sensitive to the ligand identity, but in a manner that was not fully consistent with the spectrochemical series. Excited-state relaxation of cis-Ru(dcbH2)2(NCS)2, where dcbH2 is 4,4'-(CO2H)2-2,2'-bipyridine, sometimes termed N3, anchored to mesoporous TiO2 or ZrO2 thin films immersed in CH3CN occurred through the upper MLCT excited state with activational parameters in surprisingly good agreement with those abstracted from data measured in fluid solution. An important finding from these studies is that the population of dissociative ligand field excited states is unlikely to lead to unwanted photochemistry of dye-sensitized solar cells based on cis-Ru(LL)2(NCS)2-type compounds at room temperature.

The nickel ion has a remarkably rich coordination chemistry among the first-row transition metals. Complexes with sulfur containing ligands are particularly notable, since they can manifest classical/metal-based (innocent) or inverted/ligand-based (non-innocent) behavior depending on the chemical composition of the S-ligands and the coordination geometry. Using sulfur K-edge X-ray absorption spectroscopy (XAS), we established a spectrochemical series for [Ni(II)S4] complexes containing thiolate, aliphatic dithiolate, olefinic and aromatic enedithiolate, conjugated dithiocarbamate, and aliphatic thioether ligands. The pre-edge intensities at the sulfur K-edge follow an increasing trend from tetrathiolate through dithiolate and enedithiolate to tetrathioether. In order to obtain quantitative sulfur orbital compositions from XAS data, we generalized the earlier methods of estimating the sulfur 1s → 3p transition dipole integral for a broad range of S-ligands by considering chemical shifts of spectroscopic features due to changes in the sulfur effective nuclear charge and S-ligand coordination to Ni. The XAS-based experimental orbital compositions are compared with a comprehensive set of density functional theory-based, electronic structure calculations. The 1,2-dithiolate ligands gave indication of inverted bonding, independently whether the coordinated sulfur centers are connected by constrained single or double C,C bonds. Despite intense pre-edge features, both dithiocarbamate and thioether complexes can be described with classical inorganic bonding description.

1-Methylhydantoin and its novel nickel(II) complex [Ni(H2O)4(1-mhyd)2] were prepared and identified, by elemental analysis, single crystal X-ray determination and MS methods. In addition, the complex was characterized by spectroscopic (IR, UV–Vis), magnetic and thermal techniques. The ligand reveals an interesting supramolecular architecture with both classical and non-conventional extended HB bonding networks. All rings and chains formed due to this HB bonding are embedded into the undulated pattern. A single crystal X-ray diffraction analysis of the complex shows that the nickel ion is coordinated by deprotonated hydantoin and water ligands in a N2O4 tetragonal arrangement. In the [Ni(H2O)4(1-mhyd)2] structure both inter and intramolecular hydrogen bonds are created with the participation of water molecules. The ESI-MS method confirmed mono-nuclearity of the complex while electronic spectroscopy proved the tetragonal and pseudooctahedral geometries around the metal ion in the solid state and solution, respectively. By application of the “average environment rule”, 10Dq parameters were obtained for the hypothetical, hexa-coordinate [Ni(1-mhyd)6] approximation or rather more realistic [Ni(1-mhyd)3] chelate. Based on this the mhyd ligand was ranked in the spectrochemical series close to ammonia. The general consideration of the structure of the hydantoin complexes as a function of the metal ion hardness within the framework of the HSAB theory has been provided. Both the ligand and the complex were found to be non-toxic agents against breast (MCF-7), lung carcinoma epithelial (A549) and mouse fibroblasts (Balb/3T3) cancer cell lines.

The solutions of 1:4 complexes of Co(X)(2)(D1py)(4), X = Cl(-), and NCO(-) and D1py = phenylpyridyldiazomethane, were photolyzed under cryogenic conditions, and their magnetic properties were investigated by direct current (DC) and alternating current (AC) magneto/susceptometries. After irradiation, the resulting cobalt-carbene complexes, Co(X)(2)(C1py)(4), exhibited the behaviors of heterospin single-molecule magnets (SMMs) strongly depending on the axial ligands. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), the effective activation barriers, U(eff), for the reorientation of the magnetic moment and the resonant quantum tunneling time, τ(Q), characteristic to SMM properties were estimated to be 91 and 130 K, and 4 × 10(3) and 2 × 10(5) s, respectively. The τ(Q) of Co(NCS)(2)(C1py)(4) with U(eff) = 89 K was found to be 6 × 10(2) s. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), temperature-dependent hysteresis loops were also observed below the blocking temperature (T(B) = 3.2 and 4.8 K, respectively) and the coercive forces, H(c), of 7.0 and 20 kOe at 1.9 K, respectively, were obtained. In a series of 1:4 complexes of Co(X)(2)(C1py)(4), X = NCS(-), Cl(-), and NCO(-), the axial ligands strongly affected the heterospin SMM properties, and the NCO(-) ion having the large magnitude of the ligand-field splitting in a spectrochemical series, gave the largest U(eff) and H(c) and the longest τ(Q).

The origin of the spectrochemical series and the different dependence of crystal-field splitting (10Dq) and Racah parameters on the metal-ligand distance, R, is explored through ab initio calculations on Cr(3+)-doped K2NaScF6, Cs2NaYCl6, Cs2NaYBr6, and Cs2NaYI6 lattices. For this purpose both periodic and cluster calculations have been performed. An analysis of ab initio results proves that 10Dq values mostly come from the small admixture of deep nLs ligand orbitals present in the antibonding eg(∼ x(2)-y(2),3z(2)-r(2)) level and not from the dominant covalency with valence nLp ligand orbitals, which is actually responsible for the reduction of Racah parameters. This study thus reveals the microscopic origin of the stronger dependence upon R of 10Dq when compared to that observed for Racah parameters, thus explaining why electronic transitions which are 10Dq-independent give rise to sharp optical bands. As a salient feature, while the covalency with nLp levels increases significantly on passing from CrF6(3-) to CrI6(3-), the nLs admixture in eg is found to be practically unmodified. This fact helps to understand the progressive decrease of 10Dq through the series of CrF6(3-), CrCl6(3-), CrBr6(3-), and CrI6(3-) complexes embedded in the corresponding host lattices when compared at the corresponding equilibrium distance at zero pressure. The growing importance of the nLs admixture is well-depicted using deformation density diagrams on passing from the ground state (4)A2(t2g(3)) to the (4)T2(t2g(2)eg) excited state depicted at several R values.

The synthesis of iron(II), cobalt(II) and nickel(II) complexes supported by chelating borate ligands containing one pyrazole and two thioethers, phenyl(pyrazolyl)bis((alkylthio)methyl)borates, [Ph(pz)BtR], is described. The six-coordinate complexes [Ph(pz)Bt]2M, M = Fe (1Fe), Co (1Co) and Ni (1Ni), form exclusively the cis isomers as confirmed by X-ray diffraction analyses. Whereas 1Co and 1Ni are high spin, 1Fe exhibits a room temperature magnetic moment, μeff = 4.1 μB, consistent with spin-crossover behavior. Quantitative analysis of the electronic spectrum of 2Ni leads to a value of Dq = 1086 cm−1, reflective of a ligand field strength somewhat weaker than those imposed by the related tridentate borate ligands Tp or PhTt. Replacement of the methylthioether substituent with the sterically more demanding tert-butylthioether leads to the isolation of [Ph(pz)BttBu]MX, M = Co, X = Cl (2Co); M = Ni, X = Cl (2Ni) or acac (3). The solid state structures of 2Co and 2Ni are chloride-bridged dimers. Additional high-spin cobalt(II) complexes, accessible under distinct preparative conditions, [κ2-Ph(pzH)BttBu] CoCl2·THF (4) and [κ2-Ph(pz)BttBu]2Co (5), have been fully characterized.

Mononuclear [Fe(H2LR)2]X2 (R = H, 2-Me, 5-Me, 2-Et-5-Me; X = ClO4, BF4) and dinuclear [Fe2(H2LR)3]X4 complexes containing imidazole-4-carbaldehyde azine (H2LH) and its derivatives prepared by condensation of 4-formylimidazole, 2-methyl- or 5-methyl-4-formylimidazole, or 2-ethyl-4-methyl-5-formylimidazole, with hydrazine in a 2:1 mole ratio in methanol, were prepared and their magnetostructural relationships were studied. In the mononuclear complexes, H2LR acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H2LR acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate structure. At room temperature, [Fe2(H2LH)3](ClO4)4 and [Fe2(H2L2-Me)3](ClO4)4 were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H2L2-Me with electron-donating methyl groups being stronger than H2LH, with the order of the ligand field strengths being H2L2-Me > H2LH. However, in the mononuclear [Fe(H2LH)2](ClO4)2 and [Fe(H2L2-Me)2](ClO4)2 complexes, a different order of ligand field strengths, H2LH > H2L2-Me, was observed because [Fe(H2LH)2](ClO4)2 was in the LS state while [Fe(H2L2-Me)2](ClO4)2 was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H2L2-Me ligand and the other ligand in [Fe(H2L2-Me)2](ClO4)2 is responsible for the observed change in the spin state. The same is true for [Fe(H2L2-Et-5-Me)2](ClO4)2, while [Fe(H2L5-Me)2](ClO4)2 does not involve such a steric congestion and stays in the LS state over the temperature range 5–300 K. Two kinds of crystals (polymorphs) were isolated for [Fe2(H2LH)3](BF4)4 and [Fe2(H2L2-Et-5-Me)3](ClO4)4, and they exhibited different magnetic behaviors.

Mononuclear [Fe(H(2)L(R))(2)](2+) and dinuclear [Fe(2)(H(2)L(R))(3)](4+) (R = H, 2-Me, 5-Me) complexes containing the new imidazole-4-carbaldehyde azine ligand (H(2)L(H)) and its derivatives (H(2)L(2-Me) and H(2)L(5-Me)) prepared from the condensation reaction of 4-formylimidazole or 2-methyl- or 5-methyl-4-formylimidazole with hydrazine (2:1) were prepared, and their magnetostructural relationships were studied. In the mononuclear complexes, H(2)L(R) acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H(2)L(R) acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate. At room temperature, [Fe(2)(H(2)L(H))(3)](ClO(4))(4) and [Fe(2)(H(2)L(2-Me))(3)](ClO(4))(4) were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H(2)L(2-Me) with electron-donating methyl groups being stronger than H(2)L(H), with the order of the ligand field strengths being H(2)L(2-Me)H(2)L(H). However, in the mononuclear [Fe(H(2)L(H))(2)](ClO(4))(2) and [Fe(H(2)L(2-Me))(2)](ClO(4))(2) complexes, a different order of ligand field strengths, H(2)L(H)H(2)L(2-Me), was observed because [Fe(H(2)L(H))(2)](ClO(4))(2) was in the LS state while [Fe(H(2)L(2-Me))(2)](ClO(4))(2) was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H(2)L(2-Me) ligand and the other ligand in [Fe(H(2)L(2-Me))(2)](ClO(4))(2) is responsible for the observed change in the spin state. Two kinds of crystals, needles and blocks, were isolated for [Fe(2)(H(2)L(H))(3)](BF(4))(4), and both exhibited a sharp spin transition, [LS-HS]--[HS-HS]. The spin transition of the block crystals is more abrupt with a hysteresis, T(c) upward arrow = 190 K and T(c) downward arrow = 183 K with DeltaT = 7 K.

We have studied a novel universal spectrochemical series of the six-coordinated octahedral 3d transition metal complexes that can be used for any combinations of the central metal atoms and the ligand molecules. The spectrochemical series for the six-coordinated octahedral metal complexes can be well explained by the energy level comparison between the metals and ligands using the DV-Xα method. The two-dimensional spectrochemical series can be applied for expecting of the ligand field splitting energy widths of not only the known compounds but also the unknown ones. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

The spectrochemical and nephelauxetic series are analyzed within the context of spin-polarized conceptual DFT. For a series of different [RuL(6)](x) complexes, the local spin-philicity omega(S,Metal)+ condensed on the metal ion shows a remarkable analogy with some semi-empirical scales of the spectrochemical series. The local f(SS,Metal)+ Fukui function in turn can be linked to the nephelauxetic effect. Herein, we present a non-empirical, unified approach for a quantitative discussion of both series.

Three new tetranuclear compounds of formula [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](PF(6))(4) x G where L = tris(pyridin-2-ylmethyl)amine (TPMA) [G = 0] (1), (6-methylpyrid-2-ylmethyl)-bis(pyrid-2-ylmethyl)amine (MeTPMA) [G = 0] (2), or bis(6-methylpyrid-2-ylmethyl)-(pyrid-2-ylmethyl)amine (Me(2)TPMA) [G = NH(4)PF(6)] (3) and phen = 1,10-phenanthroline) have been synthesized and characterized. The three compounds crystallize in the C2/c space group and consist of [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](4+) square shaped cations with two distinct iron(II) sites. The sites, associated with [Fe(phen)(2)(CN)(2)] and [Fe(L)(NC)(2)] moieties, are connected by cyanide bridging ligands and reside in different [FeN(4)C(2)] and [FeN(6)] ligand field strength environments. For 1, the structural features of both sites at 100 and 293 K are those of an iron(II) atom in the low-spin state, according to the magnetic properties. At 370 K the structure of the [FeN(6)] site is consistent with a quite complete change of spin state from the low-spin state to the high-spin state, a behavior confirmed by the magnetic study. Introduction of a methyl substituent in the sixth position of one or two pyridine groups to get MeTPMA or Me(2)TPMA derivatives, respectively, induce in 2 and 3 notable steric constraint in the [FeN(6)] site making longer the average Fe-N bond distances thereby weakening the ligand field strength and stabilizing the high-spin state. The [FeN(4)C(2)] site remains in the low-spin state in the three compounds.

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.

We have synthesized a series of M(IPSO) 3 (BPh 4 ) 2 complexes consisting of first row transition metals and a photochromic phenanthroline ligand: (M = Mn(II), Fe(II), Co(II), Ni(II), Zn(II), and Cu(II), IPSO = spiro[indole-phenanthrolinoxazine]). The optical properties associated with photochromic behavior were evaluated by determination of the (i) equilibrium constants ( K ) for the thermal and photostationary states, (ii) rates of photochemical ring-opening and ring-closure, and (iii) rates of thermal ring-closure. The photocolorability values associated with conversion from the colorless spirooxazine to the colored photomerocyanine isomer are greatly enhanced upon metal coordination. Variable temperature magnetic susceptibility experiments suggest deviations from cubic symmetry associated with desymmetrization induced by a dependence of ligand field strength on the photochromic state of IPSO.

A quantum chemical study based on the density functional theory (DFT) on anionic and cationic bis(phthalocyaninato)lanthanides revealed that removal of two electrons from the anionic complex shortens considerably the separation between phthalocyaninato (Pc) ligands. This suggests that [Pc2Tb]+, which is generated by two-electron oxidation from the [Pc2Tb]− SMM previously reported, can have significantly larger ligand field splitting than the original anionic form.