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The magnetic susceptibility of the dinuclear chromium(III) complex[(CH 3 CN) 5 CrOCr(NCCH 3 ) 5 ](BF 4 ) 4 ·2 CH 3 CN has been measured and analyzed. With a fitted value of the triplet energy J = 650 cm −1 , the antiferromagnetic coupling is the strongest hitherto determined for an unsupported linear oxide-bridged dinuclear Cr(III) system. The applicability of DFT for computational prediction of exchange in strongly coupled chromium(III) dimers was examined and an optimal and accurate modeling approach was devised. By such modeling it was shown possible to reproduce experimental exchange coupling constants with small relative errors typically of less than 10% ranging from the strongest coupled systems to systems with moderately strong couplings. A significant influence (>20%) of the chemical nature of the peripheral, non-bridging ligands on the exchange coupling was found and rationalized.

The reaction of [MnIII(5-Brsalen)(MeOH)](ClO4) (5-Brsalen2 − = N,N′-ethylenebis(5-bromosalicylidene-iminate)) with [IrIII(CN)6]3 − yields a trinuclear, cyanide-bridged MnIII–IrIII–MnIII cluster which is a member of an isostructural series of MnIII–MIII–MnIII clusters. The presence of the large, diamagnetic [IrIII(CN)6]3 − bridging unit facilitates a precise determination of the MnIII zero-field splitting (zfs) parameters by the combined use of inelastic neutron scattering, high-field, high-frequency EPR spectroscopy and magnetic measurements. The single-ion axial (D) and rhombic (E) zfs parameters are found to be D = − 3.72(5) cm− 1 and |E| = 0.21(1) cm− 1. The experimental data are consistent with a small, antiferromagnetic exchange interaction between the two MnIII ions.

The reaction of diamagnetic trans-[ReV(O)2(py)4]NO3 with [Ln(hfac)3(H2O)2] (Ln = Dy, Y, Tb; hfac− = 1,1,1,5,5,5-hexafluoroacetylacetonate(1–)) in chloroform yields oxide-bridged, trinuclear [Ln(hfac)3(H2O)–Re(O)2(py)4–Ln(hfac)3(NO3)] (DyReDy, YReY, TbReTb). The series is isostructural to a fluoride-bridged {Dy–Cr–Dy} complex previously reported (Dreiser et al., Chem. Sci. 3 (2012) 1024) but, notably, DyReDy exhibits frequency-dependent out-of-phase ac susceptibility (χ″) signals at higher temperatures than the chromium analogue. Magnetic investigations of a solid solution of Dy3+ in YReY reveal the presence of a small Dy–Dy exchange interaction in DyReDy.

Interaction of robust chromium(III) fluoride complexes with sodium or lithium cations in solution lead to hypsochromic spectral shifts of increasing magnitude along the series: trans-[CrF2(py)4]+, mer-[CrF3(terpy)], and fac-[CrF3(Me3tacn)]. Crystalline products isolated from solution exhibit μ2-bridging by the fluoride ligands in a linear fashion between Na+-ions and chromium centres in catena-[Na(H2O)4(μ-F)-trans-{CrF(py)4}](HCO3)2 and in the dimers [Li(H2O)n(μ-F)-trans-{CrF(py)4}]2+ (n = 3, 4). The uncharged chromium complexes fac-[CrF3(Me3tacn)] and mer-[CrF3(terpy)] have been synthesized from mer-[CrF3(py)3] and shown to precipitate sodium salts from solution, of which 3[CrF3(Me3tacn)]·2Na(Bph4)·solv and 6[CrF3(terpy)]·4Na(Bph4)·solv have been crystallographically characterized. In these clusters, the neutral fluoride complexes bring the Na+ cation separation down to 3.610 A and 3.369 A, respectively, which is much closer than the inter-cation distance in NaCl and comparable to that of NaF. DFT calculations support the notion of a strong interaction between Na+ ions and neutral chromium(III) fluoride complexes. The calculations reproduce the magnitude and the counter-intuitive sign of the spectral shifts induced by second sphere complexation in solution, which originates in a breakdown of the assumption of parameter transferability in ligand-field descriptions.

The syntheses, structures and magnetic properties of five new manganese (III) cyclam complexes, trans-[Mn(cyclam)(OH2)2](CF3SO3)3 AE H2O, trans-[Mn(cyclam)I2]I, trans-[Mn(cyclam)(ONO)2]ClO4, trans-[Mn(cyclam)(OClO3)2]ClO4 and trans-[Mn(cyclam) (CH3COO)(CH3COOH)](ClO4)2, are reported. Cyclam is the tetradentate amine ligand 1,4,8,11-tetraazacyclotetradecane. The complexes all exhibit pronounced tetragonal elongation of the coordination octahedron with the four cyclam nitrogens occupying the four equatorial positions. The magnetic properties are consistent with the formulation of the complexes as high-spin d 4 systems. trans-[Mn(cyclam)(OH2)2](CF3SO3)3 AE H2O is shown to be a convenient starting material for the syntheses of trans cyclam complexes. [Mn(cyclam)(CH3COO)(CH3COOH)](ClO4)2 exhibits extremely short intermolecular hydrogen bonds resulting in a pseudo-chain structure. The tilt of the axial ligands with respect to the equatorial plane containing the manganese and the cyclam nitrogen atoms is discussed. � 2004 Elsevier B.V. All rights reserved.

Inelastic neutron scattering (INS) data are presented for the salt (ND 4 ) 2 Cr(OD 2 ) 6 (SO 4 ) 2 , which enable the zero-field-splitting of the 5 A g ( C i ) ground term of the [Cr(OD 2 ) 6 ] 2+ cation to be defined in the temperature range 1.5–297 K. Above ∼100 K, the energies of the INS transitions are strongly temperature-dependent, as are the documented Cr–O bond lengths. The experimental data are interpreted using a 5 E⊗e Jahn–Teller Hamiltonian perturbed by low symmetry strain. Good agreement is obtained using a parameter set analogous to that employed to describe the electronic and molecular structure of the [Cu(OD 2 ) 6 ] 2+ cation in the isostructural copper(II) salt.

The correlation between the stereochemistry and electronic structure of the [V(OH 2 ) 6 ] 3+ ] cation has been examined using a variety of physical techniques in conjunction with angular overlap model calculations. The experimental data includes the first reported high-field, high-frequency EPR study of a vanadium(III) complex which enables a precise determination of the ground term spin-Hamiltonian parameters. The electronic structure and vibrational spectrum of the [V(OH 2 ) 6 ] 3+ cation was studied in samples formed from the co-crystallization of RbV(SO 4 ) 2 ·12H 2 O and RbGa(SO 4 ) 2 ·12H 2 O. The structural modifications of these salts differ in terms of the orientation of the water molecules about the tervalent cation while the M III O 6 framework remains approximately octahedral. The electronic structure of the [V(OH 2 ) 6 ] 3+ cation in samples of Rb[Ga:V](SO 4 ) 2 ·12H 2 O is found to depend greatly on the relative proportions of gallium(III) and vanadium(III). The experimental data are in accordance with predictions based on the angular overlap model when the π -bonding normal to the plane of the water molecule is dominant over the in-plane interaction ( e π ⊥ − e π∥ ca. 930 cm −1 for trigonal planar water coordination). The π -anisotropy results in a large trigonal field splitting of the 3 T 1 g ( O h ) ground term in RbV(SO 4 ) 2 ·12H 2 O (1930 cm −1 ) which diminishes almost to zero when [V(OH 2 ) 6 ] 3+ is doped into RbGa(SO 4 ) 2 ·12H 2 O, on account of the change in the orientation of the water molecules imposed by hydrogen bonding constraints. This work demonstrates the strong correlation between the stereochemistry and electronic structure of the [V(OH 2 ) 6 ] 3+ cation and accounts for the structural abnormalities reported for vanadium(III) salts of this type.

The exchange splittings in the ground and ligand-to-metal charge transfer (LMCT) states of the dimer [Cr2(OH)3(tmtame)2]3+ (tmtame=1,1,1–tris(aminomethyl)ethane) are derived from optical absorption spectroscopy. The ground-state splitting is antiferromagnetic, whereas the energy ordering of the lowest observed LMCT states is ferromagnetic. This is a new situation. It is due to the dominant direct interactions between metal-centered orbitals, which leads to a double-exchange situation in the LMCT electron configurations.

By using classical ideas to exchange theory we have found that for some specific physical situations kinetic exchange, the second-order effect of virtual electron transfer between magnetic ions, can give rise to strong ferromagnetic interactions. This is in contradiction to earlier belief.

The synthesis and characterization of a tetranuclear chromium(III) complex of the ligand 1,3,5-triaminocyclohexane, (tach), C6H15N3, are described. The complex [Cr4(tach)4(OH)6] (ClO4)n(CF3SO3)6−n, where n refines to approximately 3.57, has been isolated and its structure has been determined from three-dimensional crystallographic data. The complex crystallizes in space group P21/c of the monoclinic system with four tetranuclear species in a cell of dimensions a=22.365(4), b=12.581(3), c=22.508(5) A, β=101.75(3)°. The structure has been refined to a final R factor of 0.0737 based on 6023 observed independent reflections. Each chromium(III) center is pseudo-octahedrally bonded to the three nitrogen atoms of the ligand and to three hydroxo groups; each hydroxo group acts as a bridge between two chromium atoms. The complex adopts the adamantane structure in which all four chromium atoms are chemically equivalent, as are all six hydroxide groups. The six independent Cr···Cr separations in the tetramer are in the narrow range 3.660–3.685 A. The temperature dependence of the magnetic susceptibility of the complex is consistent with the observed structure, the tetranuclear complex exhibiting an antiferromagnetic interaction with a J value of 8.91(2) cm−1. A model is presented which makes it possible to compare the antiferromagnetic interaction in complexes containing different numbers of metal atoms.

The synthesis and characterization of di-μ-oxodimanganese(III,IV) and (IV, IV) complexes employing the tetradentate ligand 1,4,7,10-tetraazacyclododecane (cyclen) are reported. The crystal structure of the Mn(III,IV) complex, di-μ-oxo-bis(1,4,7,10-tetraazacyclododecane)dimanganese(III,IV) chloride lithium chloride pentahydrate, [(cyclen)Mn(III)O 2 Mn(IV)(cyclen)]Cl 3 ·LiCl·5H 2 O( 1a ), has been established by three dimensional X-ray diffraction techniques. The complex, of formula Mn 2 C 16 H 50 N 8 O 7 Cl 4 Li, crystallizes in the monoclinic space group C2/c with four molecules in a cell of dimensions a =20.284(4), b =10.576(2), c =15.345(2) A and β=105.70(1)°. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0344 based on 3445 observed independent intensities. The dimer has a crystallographic inversion center in the middle, which is due to a static disorder in the crystals. The Mn-Mn distance in the complex is 2.694(1) A and the bridging Mn-O-Mn angle is 95.7(1)°. The magnetic properties of the III/IV complex are consistent with a doublet ground state, the observed J value of -138(2) cm −1 for la and -156.5(5) cm −1 for the corresponding perchlorate salt being similar to those observed in other III/IV complexes. The Mn(IV,IV) complex has a singlet ground state, with J =-144.5(10) cm −1 The EPR spectrum of the III/IV complex exhibits the commonly-occurring sixteen line pattern, with a significant degree of anisotropy. The EPR spectrum has been simulated by diagonalization of the energy matrix for the doublet state. The complex exhibits two quasi-reversible waves in the cyclic voltammogram, with redox potentials of 0.069 and 0.846 V (versus Ag/AgCI) for the III/III↔III/IV and III/ IV↔IV/IV couples, respectively. The electronic spectra of both the (III,IV) and (IV,IV) forms of the complex are similar to those of analogous complexes.