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The conformational structures and properties of the anticancer drug docetaxel (DTX) are studied theoretically. A total of 3888 trial structures were initially generated by all combinations of internal single-bond rotamers and screened with the B3LYP/3-21G* method. A total of 31 unique conformers were further optimized at the B3LYP/6-311G* method. Their relative energies, dipole moments, rotational constants, and harmonic vibrational frequencies were predicted. Single-point relative energies were then determined at the M06-L/6-311G(2df,p) level. The UV spectrum of the lowest-lying DTX conformer in methanol was investigated with the TD-CAM-B3LYP/6-311 + G(2df,p) method. The 31 unique DTX structures are mainly docked at three different sites within β-tubulin. Based on the results of molecular docking and double-float MD simulations, the lowest-lying DTX conformer consistently exhibits good docking performance with β-tubulin. We identified the residues LYS299, ARG215, GLN294, LEU275, THR216, GLU290, PRO274, and THR276 on β-tubulin as active sites forming a binding pocket responsible for locking DTX within β-tubulin to make the combination more stable. The RMSD values show that the predicted complexes are favorable, and the SASA analysis shows that the hydrophilic properties of DTX are better than paclitaxel. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

Trinuclear and tetranuclear ruthenium carbonyls of the types Ru3(CO)n(NO)2, Ru3(N)(CO)n(NO), Ru3(N)2(CO)n, Ru3(N)(CO)n(NCO), Ru3(CO)n(NCO)(NO), Ru4(N)(CO)n(NO), Ru4(N)(CO)n(NCO), and Ru4(N)2(CO)n related to species observed experimentally in the chemistry of Ru3(CO)10(µ-NO)2 have been investigated using density functional theory. In all cases, the experimentally observed structures have been found to be low-energy structures. The low-energy trinuclear structures typically have a central strongly bent Ru–Ru–Ru chain with terminal CO groups and bridging nitrosyl, isocyanate, and/or nitride ligands across the end of the chain. The low-energy tetranuclear structures typically have a central Ru4N unit with terminal CO groups and a non-bonded pair of ruthenium atoms bridged by a nitrosyl or isocyanate group. [ABSTRACT FROM AUTHOR]

In density cumulant functional theory (DCFT) the electronic energy is evaluated from the one-particle density matrix and two-particle density cumulant, circumventing the computation of the wavefunction. To achieve this, the one-particle density matrix is decomposed exactly into the mean-field (idempotent) and correlation components. While the latter can be entirely derived from the density cumulant, the former must be obtained by choosing a specific set of orbitals. In the original DCFT formulation [W. Kutzelnigg, J. Chem. Phys. 125, 171101 (2006)] the orbitals were determined by diagonalizing the effective Fock operator, which introduces partial orbital relaxation. Here we present a new orbital-optimized formulation of DCFT where the energy is variationally minimized with respect to orbital rotations. This introduces important energy contributions and significantly improves the description of the dynamic correlation. In addition, it greatly simplifies the computation of analytic gradients, for which expressions are also presented. We offer a perturbative analysis of the new orbital stationarity conditions and benchmark their performance for a variety of chemical systems. [ABSTRACT FROM AUTHOR]

The first‐row transition metal 1‐norbornyl derivatives CpM(nor) Cp = η5‐C5H5; nor = 1‐norbornyl; M = Ti to Cu), including the experimentally known CpNi(nor) reported by Lehmkuhl and Dimitrov, have been examined by density functional theory and benchmarked by the more accurate DLPNO‐CCSD(T) method. The lowest energy CpM(nor) structures for the later first row transition metals from manganese to copper are found to be high‐spin state structures in which the 1‐norbornyl ligand is bonded to the metal only through the M–C σ‐bond to the bridgehead carbon atom. The favored spin states range from doublet for CpCu(nor) to sextet for CpMn(nor). The pattern of favorable structure types changes for the CpM(nor) derivatives of the earlier first row transition metals to the left of manganese since all of their low energy structures have singly or doubly agostic 1‐norbornyl groups. The agostic C–H–M interactions in the CpM(nor) derivatives are characterized by their reduced density gradients and low ν(C–H) frequencies. [ABSTRACT FROM AUTHOR]

The 1-norbornyl (nor) ligand is known experimentally to form stable transition metal alkyl derivatives through direct metal–carbon bond formation. This appears to be related to its steric bulk and inaccessibility towards β-hydrogen elimination, as exemplified by the tetraalkyls (nor)4M, some of which are very stable. In this connection we have used density functional theory and the DLPNO-CCSD(T) method to investigate the 1-norbornylcobalt carbonyl derivatives (nor)Co(CO)n (n = 4, 3, 2, 1) and (nor)2Co2(CO)n (n = 7, 6, 5). Low-energy structures of the unsaturated systems (nor)Co(CO)n (n = 3, 2) and (nor)2Co2(CO)n (n = 6, 5) are found to have agostic hydrogen atoms from a CH2 group adjacent to the Co–C bond. Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than ∼2 Å. In such structures unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH2 group. In addition, structures in which carbonyl migration from cobalt to carbon has occurred to form acyl norCO ligands are among the lowest energy structures. The resulting acyl carbonyl groups of the norCO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety. Furthermore, such neutral norCO acyl ligands can either be one-electron donors to a cobalt atom, bonding solely through the carbonyl carbon, or three-electron donor η2-μ-norCO groups bridging a central Co2 unit through both the acyl carbon and oxygen atoms. The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient (RDG) values. [ABSTRACT FROM AUTHOR]

The structures and energetics of the bis(butadiene) complexes of the second row transition metals (C4H6)2M) M=Zr to Pd) have been investigated by density functional theory. The energetically accessible (C4H6)2M structures (M=Zr to Rh) are found to have only tetrahapto η4‐C4H6 ligands. The second row transition metals energetically prefer low spin states in their bis(butadiene) derivatives (C4H6)2M reflecting their increased ligand field strength relative to the first row transition metals. Thus the energies of the singlet and doublet spin states for (C4H6)2Ru and (C4H6)2Tc, respectively, are lower than their triplet and quartet spin state isomers in contrast to the corresponding iron and manganese systems, respectively. Furthermore, the staggered maximum spin (C4H6)2M structures with χ ≈ 90° having tetrahedral coordination of the central metal to the four C=C double bonds of the two butadiene ligands found as lowest energy structures for the first row transition metals are not found for the second row transition metals from Mo to Pd. Instead the lowest energy (C4H6)2M structures for the second row transition metals have an eclipsed confirmation with χ ≈ 0° implying square planar coordination of the central metal. The lowest energy structure of the palladium complex (C4H6)2Pd is anomalous since only one of the two butadienes is a tetrahapto ligand whereas the second butadiene is only a dihapto ligand leaving one uncomplexed C=C double bond. [ABSTRACT FROM AUTHOR]

Density functional theory studies show that the lowest energy C4F8Fe(CO)4 structure is not the very stable experimentally known ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4 obtained from Fe(CO)12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO)4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol−1. However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO)4 complexes from tetrafluoroethylene are very high (e.g., ∼70 kcal mol−1). Therefore the ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4, which does not require a fluorine shift to form from Fe3(CO)12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C4F8)2Fe2(CO)n (n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands. [ABSTRACT FROM AUTHOR]

The known sandwich compound [η5-(CH2)3N2(BPh)2CMe]2Fe in which adjacent C2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B3N3H6, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me4B2N2CH)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me4C5H)2M. Low-energy sandwich structures having parallel B2N2C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η5-Me4B2N2CH)2M sandwich compounds relative to the corresponding metallocenes (η5-Me4C5H)2M. Thus (η5-Me4B2N2CH)2Cr has a quintet ground state in contrast to the triplet ground state of (η5-Me4C5H)2Cr. Similarly, the sextet ground state of (η5-Me4B2N2CH)2Mn lies ∼18 kcal mol−1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me4C5H)2Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η5-(CH2)3N2(BPh)2CMe]2Fe. [ABSTRACT FROM AUTHOR]

The unexpected 2004 discovery of decamethyldizincocene suggested some new possibilities for organometallic compounds in which an M2 unit is sandwiched between two planar carbocyclic rings. Density functional theory shows that the low-energy structures for the dizincocenes Zn2(C5X5)2 (X = H, F) are singlet coaxial structures having two (η5-C5X5)Zn units linked by a Zn–Zn single bond of length ∼2.3 Å. However, the low-energy M2(C5H5)2 (M = Cu, Ni, Co, Fe) structures have perpendicular configurations with bridging C5H5 ligands. They exhibit increasingly higher spin states from singlet (Cu, Ni) to triplet (Ni,Co), quintet (Co,Fe), and septet (Fe). The low-energy structures of the perfluorinated systems M2(C5F5)2 (M = Co, Fe) have irregular geometries with one bent bridging C5F5 ring and one planar terminal pentahapto η5-C5F5 ring, with the metal atom bearing the terminal η5-C5F5 ligand has the favored 18-electron configuration while the other metal atom has only a lower electron configuration and vacant coordination sites. [ABSTRACT FROM AUTHOR]

Density functional theory has been used to examine the dimetallocene-like dicycloheptatrienyl dimetal compounds of the second-row transition metals (C7H7)2M2 (M5Ru, Tc, Mo, Nb, Zr). The lowest energy (C7H7)2Mo2 structure is a coaxial structure with terminal η7-C7H7 rings, whereas the lowest energy (C7H7)2M2 structures (M5Ru, Tc, Nb, Zr) are perpendicular structures with bridging η4,η4-7H7 rings except for the perpendicular (η4,η3-C7H7)2Ru2 structure. The metal- metal bond orders in the (C7H7)2M2 structures (M5Ru, Tc, Mo, Nb), as determined by analysis of their frontier molecular orbitals, suggest preferred 16- rather than 18-electron configurations for the central metal atoms. Thus, in the coaxial (57-C7H7)2M2 structures the formal bond orders are two for M = Tc and three for M = Mo. For the perpendicular structures both (η4,η3-C7H7)2Ru2 and (η4,η4-C7H7)2Tc2 have 16-electron configurations with metal-metal single bonds owing to the different modes of bonding of the bridging C7H7 rings in the two structures. For the (C7H7)2Zr2 system the perpendicular structure has a formal Zr=Zr double bond and the coaxial structure has a very long (~3.5 Å) Zr-Zr bond indicating only 12- to 14-electron configurations for the zirconium atoms. [ABSTRACT FROM AUTHOR]

Reaction of the anionic N-heterocyclic dicarbene (NHDC), [:C{[N(2,6-Pri2C6H3)]2CHCLi}]n (1), with SiCl4 gives the trichlorosilyl-substituted (at the C4 carbon) N-heterocyclic carbene complex (7). Abnormal carbene–SiCl4 complex (8) may be conveniently synthesized by combining 7 with HCl·NEt3. In addition, 7 may react with CH2Cl2 in warm hexane, giving the abnormal carbene-complexed SiCl3+ cation (9). The nature of the bonding in 9 was probed with complementary DFT computations. [ABSTRACT FROM AUTHOR]

The opposed and parallel structures for the binuclear bis(azulene) 'submarine' sandwiches (C10H8)2M2 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been optimized using density functional theory. The lowest energy (C10H8)2M2 structures of the early transition metals Ti, V, Cr, and Mn have the azulene units functioning as bis(pentahapto) ligands to each metal atom similar to the azulene ligand in the long-known molybdenum carbonyl complex (η5,η5-C10H8)Mo2(CO)6. The metal-metal bonds in these early transition metal structures have distances and Wiberg bond indices consistent with the formal bond orders required to give each metal atom an 18-electron configuration for the singlet structures and a 17-electron configuration for the triplet structures. For the later transition metals Fe, Co, and Ni, the lowest energy (C10H8)2M2 structures contain pentahapto-trihapto azulene ligands with an uncomplexed CC double bond, similar to that in the long-known iron carbonyl complex (η5,η3-C10H8)Fe2(CO)5. The parallel (η5,η3-C10H8)2M2 (M = Fe, Co, Ni) structures contain metallocene subunits with their metal atoms at long nonbonding distances of 3.5-3.9 Å from the other metal atom, which is located between the azulene C7 rings. Higher energy opposed (C10H8)2Fe2 structures contain an unprecedented distorted η6,η4-azulene ligand using six carbon atoms for bonding to one iron atom as a hexahapto fulvene ligand and the remaining four carbon atoms for bonding to the other iron atom as a tetrahapto diene ligand. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Experimental evidence is reported for the first intermediate in the classic SEAr reaction of benzene nitration with mixed acid. The UV/Vis spectroscopic investigation of the reaction showed an intense absorption at 320 nm (appearing as a band shoulder) arising from a reaction intermediate. Our theoretical modeling shows that the interaction between the two principal reactants with solvent (H2SO4) molecules significantly affects the structure of the initial complex. In this complex, a larger distance between the aromatic ring and nitronium ion precludes the possibility for electronic charge transfer from the benzene π-system to the electrophile. The computational modeling of the potential energy surface reveals that the reaction favors a stepwise mechanism with intermediate formation of π- and σ- (arenium ion) complexes. [ABSTRACT FROM AUTHOR]

The geometries, proton affinities, and relative energies of protonated digermane (Ge2H7+), distannane (Sn2H7+), and diplumbane (Pb2H7+) have been investigated by density functional theory using the correlation-consistent cc-pV nZ-PP basis sets ( n = D, T, Q). The results of the caluclations are consistent with the very limited experimental and theoretical results that are available. The lowest-lying structure of Ge2H7+ is predicted to have C2 symmetry with a bent 3-center-2-electron Ge-H-Ge bridge, analogous to Si2H7+. For Sn2H7+ and Pb2H7+, the lowest-lying structures are predicted to be different, with a D3 d structure for Sn2H7+ and a C1 structure for Pb2H7+. The predicted proton affinities decrease in the order Pb2H6 (9.84 eV) > Sn2H6 (8.48 eV) > Ge2H6 (8.11 eV) > Si2H6 (7.72 eV) > C2H6 (6.18 eV). [ABSTRACT FROM AUTHOR]

No stable germanetellone was described until Tbt(Dis)Ge=Te and Tbt(Tip)Ge=Te (Tbt = 2,4,6-tris[bis(trimethylsilyl) methyl]phenyl, Dis = bis(trimethylsilyl)methyl and Tip = 2,4,6-triisopropylphenyl) were reported in 1997. Following these initial experiments, there has arisen considerable interest in Ge=Te systems. An obvious question is: why have the simple XYGe=Te (X, Y = H, F, Cl, Br, I and CN) molecules not yet been isolated? In view of the present situation, theoretical information may be of great help for further advances in germanetellone chemistry. A systematic investigation of the XYGe=Te molecules is carried out using the second order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT). The structures and energetics, including ionization potentials (IPad and IPad(ZPVE)), four different forms of neutral-anion separations (EAad, EAad(ZPVE), VEA and VDE) and the singlet-triplet gaps, are reported. The electronegativity (χ) reactivity descriptor for the halogens (F, Cl, Br and I) and the natural charge separations of the Ge=Te moiety are used to assess the interrelated properties of germanetellone and its derivatives. The results are analyzed, discussed and compared with analogous studies of telluroformaldehyde, silanetellone and their derivatives. The thermodynamic viabilities of some of the novel germanetellones have also been evaluated in terms of the bond dissociation enthalpies of Tbt(Dis)Ge=Te and Tbt(Tip)Ge=Te. The simple mono-substituted germanetellones appear to be slightly more thermodynamically favored than Tbt(Dis)Ge=Te and Tbt(Tip)Ge=Te, since the bond dissociation enthalpies of these kinetically stabilized germanetellones are about 28 and 51 kcal mol-1 lower, respectively. [ABSTRACT FROM AUTHOR]

Although evanescent at best, the bare dimers of the elements Cr, Mo, and W have been identified as possible candidates for the sextuple metal–metal bond. The corresponding dibenzene sandwich compounds Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, satisfy the ‘18-Electron Rule’, and might achieve high-order metal–metal bonds and longer lifetimes at the same time. Twenty-two different DFT methods have been used to evaluate this possibility. Based on the present Wiberg bond index and molecular orbital analyses, however, only quadruple metal–metal bonds are predicted for the electronic ground states of Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, rather than the sextuple or even quintuple bonds, for both singlet and triplet electronic states. It is possible to force the hypothesisedD6hsextuple bond electron configuration, but the resulting energy is 39 kcal/mol above theD2hquadruple bond structure for Cr2(C6H6)2. However, the constrained sextuple bond structure for Mo2(C6H6)2lies 19 kcal/mol above the MoMo singlet. For W2(C6H6)2the sextuple bond structure is predicted to lie only 3 kcal/mol above the WW structure. Thus the answer to the question raised in our title is ‘almost’ for the tungsten–sextuple bond. [ABSTRACT FROM PUBLISHER]

The mixed sandwich compounds (η5-C5H5)M(η7-C7H7) (M = Ti, V, Cr) have been known for approximately 50 years. The vanadium derivative ('trovacene') has recently been shown to be a useful precursor for the preparation of a variety of paramagnetic sandwich compounds. In this connection the structures of the complete series of the first row transition metal mixed sandwich compounds C5H5MC7H7 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. The experimentally known parallel ring sandwich structures (η5-C5H5)M(η7-C7H7) are found for the Ti, V, and Cr derivatives. Analysis of the frontier molecular orbitals of these complexes indicates that, for the metal d-orbitals, the z2 orbital is essentially non-bonding, the x2-y2 and xy orbitals are δ bonding to the seven-membered ring, and the xz and yz orbitals are π bonding to the five-membered ring. For the later transition metals, (η5-C5H5)M(η5-C7H7) structures with pentahapto seven-membered rings are preferred for iron and manganese and (η5-C5H5)M(η3-C7H7) structures with trihapto seven-membered rings are preferred for cobalt and nickel, leading to either 17- or 18-electron complexes in all cases. A higher energy parallel sandwich structure (η5-C5H5)Co(η7-C7H7) with a quartet spin state and a formal 21-electron cobalt configuration is also found for the cobalt derivative. [ABSTRACT FROM AUTHOR]

The structures of M2(CO)6 (M = Mn, Re) are of interest since a formal metal–metal quintuple bond is required to give each metal atom the favored 18-electron configuration, assuming that all six carbonyl groups are the usual two-electron donors. However, density functional theory studies on these systems provide no evidence for such structures having the very short metal–metal distances required for formal metal–metal quintuple bonds. Instead, the lowest energy structures predicted for Mn2(CO)6 have geometries consistent with a formal Mn&tbond;Mn triple bond and two four-electron donor bridging carbonyl groups, which also lead to the favored 18-electron configuration for each manganese atom. For Mn2(CO)6, there is a higher-lying structure (6–9 kcal/mol above the global minimum) with a Mn&tbond;Mn quadruple bond distance of ∼2.18 Å. Four-electron donor bridging carbonyl groups appear to be less favorable in the low-energy Re2(CO)6 structures. Thus, the lowest energy Re2(CO)6 structure by ∼12 kcal/mol has all terminal carbonyl groups and a Re&tbond;Re distance of 2.55 Å thereby suggesting a formal triple bond and a 16-electron configuration for both rhenium atoms. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Optimization of structures for the highly unsaturated binuclear rhenium carbonyl nitrosyls Re2(NO)2(CO)n (n = 5, 4) using the MPW1PW91 and BP86 methods of density functional theory indicate complicated potential energy surfaces with eight singlet and three triplet energetically accessible structures for Re2(NO)2(CO)5 and 11 singlet and six triplet energetically accessible structures for Re2(NO)2(CO)4. The lowest energy structure for Re2(NO)2(CO)5 is a singlet predicted to have two bridging nitrosyl groups and a formal Re&tbond;Re triple bond to give both rhenium atoms the favored 18-electron configuration. However, other relatively low-energy structures for Re2(NO)2(CO)5 are found with five-electron donor bridging nitrosyl groups, four-electron donor bridging carbonyl groups, and coordinatively unsaturated rheniumatoms. The lowest energy structures for Re2(NO)2(CO)4 are predicted to be unbridged structures having rhenium–rhenium distances short enough for formal triple or even quadruple bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C4H6)3M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C4H6)3Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C4H6)3M structures of the late second-row transition metals couple to form a C12H18 ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C4H6)3M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C4H6)3Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C4H6)3Cr analog relative to quintet (C12H18)Cr isomers with an open-chain C12H18 ligand. [ABSTRACT FROM AUTHOR]

The experimentally known split (3 + 2) five‐electron donor bicyclo[3.2.1]octa‐2,6‐dien‐4‐yl (bcod) ligand provides a flexible alternative to the rigid planar cyclopentadienyl (Cp) ligand. In this connection, the structures and energetics of the binuclear iron carbonyl complexes (bcod)2Fe2(CO)n (n = 4, 3, 2, 1) have been investigated by density functional theory for comparison with the corresponding Cp2Fe2(CO)n derivatives. The cis and trans doubly CO‐bridged (bcod)2Fe2(μ‐CO)2(CO)2 structures are the lowest energy tetracarbonyl structures, similar to the Cp2Fe2(CO)4 system. However, an unbridged (bcod)2Fe2(CO)4 isomer lies only ~1 kcal/mol in energy above the doubly bridged isomers. The flexibility of the bcod ligand leads to low‐energy singlet and triplet spin state structures with agostic hydrogen atoms for the unsaturated (bcod)2Fe2(CO)n (n = 3, 2, 1) systems. Analogous structures are not found in the corresponding Cp2Fe2(CO)n systems with the rigid Cp ligand. Such structures, effectively involving donation of an electron pair from an olefinic C‐H bond to an iron atom through three‐center two‐electron C‐H‐Fe bonding, are energetically competitive with isomeric structures with metal‐metal multiple bonds. [ABSTRACT FROM AUTHOR]

Metallametallocenes are derived from metallocenes by replacing a CH unit in one of the cyclopentadienyl rings by a metal moiety. They include the known compounds CpCo(C4H4Co)Cp and CpFe(C4Ph4Ni)Cp (Cp = η5-C5H5). The metallametallocenes of the type CpM(C4H4M′)Cp (M, M′ = Fe, Co, Ni) have been studied by density functional theory including the B3LYP* method of Reiher and collaborators to improve the reliability of predicted singlet-triplet and double-quartet splittings. The lowest-energy structures for the homometallic metallametallocenes CpFe(C4H4Fe)Cp and CpCo(C4H4Co)Cp are triplet- and singlet-spin-state structures, respectively, that have formal metal-metal single bonds with predicted lengths of ca. 2.4 Å. The corresponding singlet CpFe(C4H4Fe)Cp structure with a much shorter Fe=Fe double bond length of ca. 2.27 Å is a significantly higher energy structure, which lies ca. 24 kcal/mol above the triplet structure. The lowest-energy structure of the corresponding dinickel derivative CpNi(C4H4Ni)Cp is a triplet-state structure that is suggested to have insignificant bonding between the nickel atoms. The lowest-energy structures for the heterometallic derivatives CpM(C4H4M′)Cp (M ≠ M′ = Fe, Co, or Ni) are always the lowest-spin-state structure with the electron-richest metal (Ni > Co > Fe) in the metallacyclic ring. The Wiberg bond indices that were determined by Natural Bond Orbital analysis appear to be the most reliable indication of formal metal-metal bond orders. [ABSTRACT FROM AUTHOR]

The role of a widely employed additive (AgOAc) in a palladium acetate-catalyzed ortho-C-H bond activation reaction has been examined using the M06 density functional theory. A new hetero-bimetallic Pd-(μ-OAc)3-Ag is identified as the most likely active species. This finding could have far-reaching implications with respect to the notion of the active species in palladium catalysis in the presence of other metal salt additives. [ABSTRACT FROM AUTHOR]

As an illustration of a decomposition route of metal carbonyl nitrosyls by reduction of coordinated NO groups by adjacent CO groups resulting in CO 2 liberation, the Ru 3 (CO) 10 (NO) 2 system is studied by density functional theory. Intermediates for the formation of Ru 3 (CO) 9 (µ-N 2 O) and Ru 3 (CO) 8 (µ 3 -N 2) are identified. [Display omitted] The stability of some metal carbonyl nitrosyls is limited by the oxidation of CO groups by adjacent NO groups to eliminate CO 2. Theoretical studies show how intermediates in such processes can be trapped as molecular species using the central Ru 3 unit found in the stable and experimentally known Ru 3 (CO) 10 (µ-NO) 2. Two stages of such NO reduction by adjacent CO ligands have been identified in the Ru 3 system. The first stage, as demonstrated by conversion of Ru 3 (CO) 10 (µ-NO) 2 to the N 2 O complex Ru 3 (CO) 9 (µ 3 -N 2 O), involves attack of the oxygen atom of an NO group on the carbon atom of an adjacent CO group to eliminate CO 2. This leaves a reactive nitride ligand that can couple with a second NO group to form an N 2 O complex. The oxygen atom on such an N 2 O ligand can convert an adjacent CO group to CO 2 thereby leaving behind a metal dinitrogen complex. [ABSTRACT FROM AUTHOR]

The hydrogen abstraction reaction H2S + OH → H2O + SH has been studied using the “gold standard” CCSD(T) method along with the Dunning’s aug-cc-pVXZ (up to 5Z) basis sets. For the reactant (entrance) complex, the CCSD(T) method predicts a HSH···OH hydrogen-bonded structure to be lowest-lying, and the other lower-lying isomers, including the two-center three-electron hemibonded structure H2S···OH, have energies within 2 kcal/mol. The similar situation is for the product (exit) complex. With the aug-cc-pV5Z single point energies at the aug-cc-pVQZ geometry, the dissociation energy for the reactant complex to the reactants (H2S + OH) is predicted to be 3.37 kcal/mol, and that for the product complex to the products (H2O + SH) is 2.92 kcal/mol. At the same level of theory, the classical barrier height is predicted to be only 0.11 kcal/mol. Thus, the OH radical will react promptly with H2S in the atmosphere. We have also tested the performance of 29 density functional theory (DFT) methods for this reaction. Most of them can reasonably predict the reaction energy, but the different functional give quite different energy barriers, ranged from −10.3 to +2.8 kcal/mol, suggesting some caution in choosing density functionals to explore the PES of chemical reactions. [ABSTRACT FROM AUTHOR]

Structuraland energetic features of the binuclear titanium carbonylsTi2(CO)n(n= 12, 11, 10) have been examined using density functional theory.The lowest-energy Ti2(CO)12structure is a singletstructure consisting of two Ti(CO)6units linked by TiTidouble bonds of lengths 3.0–3.2 Å. A similar slightlyhigher energy triplet Ti2(CO)12structure isfound with longer Ti–Ti bonds (3.37–3.62 Å), consideredto be formal single bonds. The energy required for the dissociationof Ti2(CO)12into two Ti(CO)6fragmentsis 18.5 ± 2 kcal/mol higher than the energy required for thedissociation of V2(CO)12into two V(CO)6fragments. For the unsaturated Ti2(CO)11system, the lowest-energy structures contain a four-electron-donorbridging η2-μ-CO group and 10 terminal CO groupswith formal TiTi double bonds in the singlet structures andformal Ti–Ti single bonds in the triplet structures. Similarly,the most favored geometries for the more highly unsaturated Ti2(CO)10contain two four-electron-donor bridgingη2-μ-CO groups with formal TiTi doublebonds for the singlet structures and formal Ti–Ti single bondsfor the triplet structures. Higher-energy triplet Ti2(CO)10structures are found with two or three two-electron-donorsemibridging CO groups and formal TiTi triple bonds of length2.7–2.8 Å. [ABSTRACT FROM AUTHOR]

Densityfunctional theory confirms the experimentally known triplybridged Rh4(CO)9(μ-CO)3structureto be the lowest-energy structure. The lowest-energy structures ofthe unsaturated systems Rh4(CO)n(n= 11, 10, 9, 8) are also triply bridged structureswith central Rh4tetrahedra that can be derived from thisRh4(CO)9(μ-CO)3structure byremoval of terminal CO groups in various ways. The MM distancesin these central M4tetrahedra change very little as COgroups are lost, suggesting reluctance to form metal–metalmultiple bonds in these unsaturated systems. The natural bond orbital(NBO) Wiberg bond indices provide depth to this analysis. All of theseunsaturated systems are predicted to be highly fluxional, as two tothree isomeric structures lie within ∼4 kcal/mol of the globalminima. The Rh4(CO)8(μ-CO)2(μ4-CO) structure analogous to the lowest-energyCo4(CO)11structure with all four atoms of acentral Co4butterfly bridged by a μ-CO group ispredicted to lie ∼6 kcal/mol in energy above the lowest-energyRh4(CO)11structure. Comparisons of the relativeenergies of analogous Rh4(CO)nand Ir4(CO)nstructures indicatethat more highly bridged M4(CO)nstructures are energetically much more favorable for rhodium thanfor iridium. Dissociation energies (for loss of CO) and disproportionationenergies are reported. [ABSTRACT FROM AUTHOR]

Therelative energies of singlet and triplet metallocene, metallabenzene,and metallacyclopentadiene C10H10M isomers (M= Ni, Pd, Pt) have been examined using density functional theory.For the C10H10Ni system, the experimentallyknown triplet nickelocene (η5-Cp)2Ni isthe lowest energy isomer by ∼17 kcal/mol with respect to singletnickelocene. For the C10H10Pd system, the tripletand singlet palladocene structures have similar energies within ∼2kcal/mol. However, the singlet palladocene has a “slipped ring”(η3-Cp)2Pd structure with two trihaptoCp rings. The C10H10Pt system is different sincethe platinabenzene CpPtC5H5isomer is the lowestenergy structure. This is in accord with the synthesis of stable substituted CpPtC5H3R2platinabenzenes by Haley and co-workers[Haley, M. M.; et al. Organometallics2004, 23, 1174]. However, the slipped ring singlet platinocene (η3-Cp)2Pt to the isomeric platinocene lies only ∼2kcal/mol above the platinabenzene global minimum, so the energy barrierfor conversion of the platinabenzene must be substantial. The followinggeneral observations can be made regarding the relative stabilitiesof isomeric C10H10M (M = Ni, Pd, Pt) structures:(1) Triplet structures become less favorable energetically than isomericsinglet structures in the sequence Ni < Pd < Pt. (2) Slippedmetallocene structures with trihapto η3-Cp ratherthan pentahapto η5-Cp rings leading ultimately to16- rather than 18-electron metal configurations become increasinglyfavorable energetically in the sequence Ni < Pd < Pt. (3) Metallabenzene(η5-Cp)MC5H5structureswith pentahapto Cp rings are always more favorable energetically thanisomeric metallacyclopentadiene (η6-C6H6)MC4H4structures with hexahaptobenzene rings. [ABSTRACT FROM AUTHOR]

The tetramethyleneethane (TME) ligandis found in the η3,η3complex trans-(η3,η3-TME)Ni2Cp2and the η4,η4complex trans-(η4,η4-TME)Co2Cp2, which have both been synthesizedand structurally characterized by X-ray crystallography. The structuresof the complete series of (TME)M2Cp2derivativesof the first-row transition metals from Ti to Ni have now been investigatedby density functional theory. The experimentally known nickel andcobalt complexes are found to have closed-shell electronic groundstates. The lowest energy structure for the iron complex is the tripletspin state trans-(η4,η4-TME)Fe2Cp2structure with geometrysimilar to that of the lowest energy cobalt structure. All of thelow-energy (TME)M2Cp2structures for manganeseand the first-row transition metals to the left of manganese (Cr,V, and Ti) exhibit cis-(η4,η4-TME)M2Cp2stereochemistry, therebyproviding the possibility for direct metal–metal interactions.Vanadium is the only first-row transition metal to the left of cobaltwhere the lowest energy (TME)M2Cp2structureis a singlet spin state, suggesting limited applicability of the 18-electronrule in these systems. The frontier molecular orbitals of these cis-(η4,η4-TME)M2Cp2systems have been examined in order to provide insightregarding metal–metal bonding. Thus, low-energy cis-(η4,η4-TME)Mn2Cp2structures are found in both quintet and triplet spin stateswith formal MnMn double bonds. The lowest energy cis-(η4,η4-TME)Cr2Cp2and cis-(η4,η4-TME)Ti2Cp2structures have tripletspin states with a formal CrCr triple bond and a formal TiTidouble bond, respectively. The lowest energy cis-(η4,η4-TME)V2Cp2structureis a singlet structure with a formal VV triple bond. [ABSTRACT FROM AUTHOR]

The so-called flyover complexes (η3,1,η3,1-μ-R3R′3C6)Co2(CO)4are importantbecause of their roleas intermediates in the cobalt carbonyl-catalyzed cyclotrimerizationof alkynes. A density functional study of such flyover complexes hasled to the discovery of isomers with bent benzene rings bridging apair of cobalt atoms. Such complexes are likely to be involved inthe process of forming a carbon–carbon bond in closing thesix-carbon chain in the flyover complexes to give the correspondingarene derivatives. The relative energies of the flyover complexesand the bridging bent benzene derivatives depend on the substituentson the six-carbon chain. Thus, for [C6(CF3)6]Co2(CO)4and [1,3,6-tBu3C6H3]Co2(CO)4with bulky CF3and tert-butyl substituentsat the ends of the six-carbon chain, the experimentally known flyovercomplexes are preferred energetically over the isomeric bridging bentbenzene ring structure by 23.3 and 1.1 kcal/mol, respectively. However,for the less sterically hindered (C6R6)Co2(CO)4(R = H, CH3) and 1,3,6-(CF3)3C6H3]Co2(CO)4derivatives the bridging bent benzene ring structures arepreferred energetically over the flyover structures by 3.6 to 14.3kcal/mol, respectively. [ABSTRACT FROM AUTHOR]

Complexationof the very unstable free heptafulvene with iron carbonylsis known experimentally to lead to stable complexes, including twoisomers of (η4-C7H6CH2)Fe(CO)3as well as the binuclear (C7H6CH2)2Fe2(CO)6.Density functional theory shows that structures of the mononuclear(C7H6CH2)Fe(CO)3withtrimethylenemethane and butadiene subunits of the heptafulvene ligandbonded to the Fe(CO)3moiety have nearly equal energieswithin ∼3 kcal/mol, consistent with the experimental observationof two isomers depending upon the synthetic method. For (C7H6CH2)Fe2(CO)6a trans-η4:η4structurewith no iron–iron bond and a cis-η3:η3allylic structure with an iron–ironbond are predicted to have energies within ∼3.0 kcal/mol. Comparisonof the predicted ν(CO) frequencies for these two structureswith the experimental ν(CO) frequencies for the structurallyuncharacterized (C7H6CH2)Fe2(CO)6suggests the cis-η3:η3allylic structure for the latter. [ABSTRACT FROM AUTHOR]

Abstract: An important recent development in metal disulfide chemistry is the 2001 synthesis by Adams et al. of Mn2S2(μ-CO)(CO)6 containing both a bridging disulfide ligand and a bridging carbonyl group. During the ensuing decade this manganese complex has proven to be a key intermediate for the synthesis of diverse manganese carbonyl sulfur derivatives, including heterometallic complexes, phosphine, arsine, and thioether derivatives. The related series of Mn2S2(CO) n complexes (n =10, 9, 8, 7, 6) has now been studied by density functional theory. The lowest energy Mn2S2(CO)10 structures have two Mn(CO)5 units bridged by a symmetrical two-electron donor disulfide ligand without a metal–metal bond. This structure may be derived from dimethyl disulfide by replacement of both methyl groups with isolobal Mn(CO)5 groups. Decarbonylation of this Mn2S2(CO)10 structure gives the lowest energy Mn2S2(CO)9 structure containing an unprecedented unsymmetrical four-electron donor bridging disulfide ligand and still no metal–metal bond. However, this Mn2S2(CO)9 structure is predicted to be thermodynamically unstable with respect to disproportionation into Mn2S2(CO)10 +Mn2S2(CO)8. The lowest energy Mn2S2(CO)8 structure has a sulfur-bridging carbonyl group as well as a carbonyl group bridging an Mn–Mn bond. Removal of this sulfur-bridging carbonyl group gives the lowest energy Mn2S2(CO)7 structure. This structure corresponds to the experimental Mn2S2(μ-CO)(CO)6 structure synthesized by Adams et al., which has a carbonyl-bridged Mn–Mn single bond. For Mn2S2(CO)6, one of the two low-energy structures can be derived from this Mn2S2(μ-CO)(CO)6 structure by loss of the bridging carbonyl group. The other low-energy Mn2S2(CO)6 structure has two Mn(CO)3 groups joined by an Mn–Mn bond, which is bridged by two separate sulfide (S) ligands. [Copyright &y& Elsevier]

Abstract: The lowest energy structure for Mn2(BF)(CO)10 is predicted to consist of two Mn(CO)5 groups bridged by a BF group without a manganese–manganese bond. This structure is related to the stable compounds Mn2(μ-BX)(CO)10 (X=Cl, Br) and [(η5-C5H5)Ru(CO)2]2(μ-BF), which have been synthesized and characterized by X-ray crystallography. The following principles determine the lowest energy structures of the Mn2(BF)(CO) n (n =9, 8, 7, 6) derivatives: (1) two-electron donor bridging μ-BF groups are highly favorable; (2) four-electron donor bridging η2-μ-BF groups are not found and thus appear to be highly unfavorable. Thus the lowest energy structure of Mn2(BF)(CO)9 is a doubly bridged structure with bridging CO and BF groups, in contrast to the experimentally observed unbridged structure of Mn2(CO)10. The lowest energy structures of Mn2(BF)(CO)8 have either a four-electron donor η2-μ-CO group or a two-electron donor bridging BF group. Similarly the lowest energy structures of the more highly unsaturated Mn2(BF)(CO) n (n =7, 6) are singlet (for n =7) or triplet (for n =6) states in which the BF group is a bridging rather than a terminal ligand. [ABSTRACT FROM AUTHOR]

Abstract: Density functional theory methods (B3LYP and BP86) indicate that the preferred structures for such early transition metal derivatives are (η8-C8H8)M(η4-C8H8) (M=Ti, V, Cr) with one octahapto η8-C8H8 ring and one tetrahapto η4-C8H8 ring. In such structures only 12 of the 16 carbon atoms of the two C8H8 rings are bonded to the metal, leading to 16-, 17-, and 18-electron complexes, respectively, in accord with the experimentally known structures for the Ti and V derivatives. The preferred structures for the Mn and Fe derivatives are (η6-C8H8)M(η4-C8H8) (M=Mn, Fe) with one hexahapto and one tetrahapto C8H8 ring and thus having 17- and 18-electron configurations, respectively, in accord with experimental data on the iron complex. The lowest energy structure for the cobalt complex is (η4-C8H8)Co(η2,2-C8H8) with two different types of tetrahapto C8H8 rings and a 17-electron metal configuration. The nickel complex (C8H8)2Ni appears to prefer a structure with a 16-electron configuration and two trihapto C8H8 rings, similar to the known (η3-C3H5)2Ni rather than a bis(tetrahapto) structure with the favored 18-electron configuration. These theoretical studies indicate that in (C8H8)2M derivatives of the first row transition metals, the number of carbon atoms in the pair of C8H8 rings involved in the bonding to the central metal atom gives the metal atoms 16-, 17-, or 18-electron configurations. [Copyright &y& Elsevier]

Abstract: Theoretical studies on the known trinuclear cobalt carbonyl derivatives ECo3(CO)9 (E=CH, CF, P, As) predict structures with carbonyl groups bridging each edge of the Co3 triangle in contrast with experiment where structures with all terminal carbonyl groups are found in all cases. However, the energy differences are predicted to be rather small ranging from 4±2kcal/mol for FCCo3(CO)9 to 10±3kcal/mol for AsCo3(CO)9. The global minima for the unsaturated ECo3(CO) n (n = 8, 7, 6) derivatives generally have two (for n =8) or three (for n =7 and 6) carbonyl groups bridging the edges of the Co3 triangle. However, structures with all terminal carbonyl groups are also found in all cases as well as higher energy structures in which one of the carbonyl groups bridges all three cobalt atoms. The fluoromethinyl derivatives FCCo3(CO) n (n =9, 8, 7) are anomalous since their unbridged structures or structures with a carbonyl group bridging all three cobalt atoms are closer in energy to the doubly or triply bridged global minima than is the case for the other ECo3(CO) n derivatives. [Copyright &y& Elsevier]

Abstract: The structures and relative energies of the As2Co2(CO) n (n =6, 5, 4) derivatives are predicted by density functional theory to be analogous to those of the corresponding H2C2Co2(CO) n derivatives. Thus As2Co2(CO)6 is predicted to have three carbonyls on one cobalt atom eclipsed relative to the three carbonyls on the other cobalt atom. The corresponding As2Co2(CO)6 structure with a staggered rather than eclipsed arrangement of the Co(CO)3 units is a transition state rather than a genuine minimum. For As2Co2(CO)5 the structure in which an equatorial group is removed from the As2Co2(CO)6 structure and a singly bridged As2Co2(CO)4(μ-CO) structure are predicted to have essentially the same energies, within <2kcal/mol. A higher energy As2Co2(CO)5 structure by 9±2kcal/mol is derived from the As2Co2(CO)6 structure by removal of an axial carbonyl group. The two unbridged As2Co2(CO)5 structures correspond to those observed experimentally in the photolysis of As2Co2(CO)6 in Nujol matrices at low temperatures. In such photolysis experiments the higher energy isomer is produced initially and then converted to the lower energy isomer upon annealing. A singly bridged structure was found for As2Co2(CO)4. The analogous structure was not observed in the previous work with H2C2Co2(CO)4. However, such a H2C2Co(CO)3(μ-CO) structure is found here for the acetylene complex. This singly bridged structure is predicted to lie 1.9kcal/mol below the H2C2Co2(CO)4 4–1S structure by the BP86 method but 3.5kcal/mol above the latter by the B3LYP method. In addition to the singly bridged As2Co2(CO)4 structure, the same six unbridged structures were located for As2Co2(CO)4 that were previously found for H2C2Co2(CO)6. [Copyright &y& Elsevier]

Abstract: Density functional theory studies on a series of Cp2Co2E2 derivatives (E=S and PX; X=H, Cl, OH, OMe, NH2, NMe2) predict global minimum butterfly structures with one Co–Co bond for the “body” of the butterfly and four Co–E bonds at the edges of the “wings” of the butterfly. Tetrahedrane structures with both Co–Co and E–E bonds are higher in energy for Cp2Co2S2 and Cp2Co2(PH)2 and are not found in the other systems. This differs from the corresponding Fe2(CO)6S2 and Fe2(CO)6(PX)2 derivatives where tetrahedrane structures are predicted to be the lowest energy structures for all cases except X=NR2 and OH and such a tetrahedrane structure is found experimentally for Fe2(CO)6S2. The butterfly structures for the Cp2Co2E2 derivatives are of two types. For Cp2Co2(PX)2 (X=H, OH, OMe, NH2, NMe2) the lowest energy structures are unsymmetrical butterflies Cp2Co2(P)(PX2) with two X groups on one phosphorus atom and a lone pair on the other (naked) phosphorus atom. Related low-energy unsymmetrical butterfly Fe2(CO)6(P)(PX2) structures, not observed in previous theoretical studies, are now found for the corresponding Fe2(CO)6(PX)2 derivatives. Symmetrical butterfly singlet diradical structures with one X group on each phosphorus atom in relative cis or trans positions are also found for the Cp2Co2(PX)2 derivatives and are the global minima for Cp2Co2(PCl)2 as well as Cp2Co2S2. In all cases the cis structures are of lower energy than the corresponding trans structures. Rhombus structures having neither Co–Co nor E–E bonds are also found for all of the Cp2Co2(PX)2 derivatives but always at higher energies than the butterfly structures, ranging from 17 to 29kcal/mol above the global minima. [Copyright &y& Elsevier]

Abstract: The unsaturated Fe2C2 tetrahedrane derivatives R2C2Fe2(CO)6 (R=Ph, t Bu) are among the many products obtained from reactions of the alkynes RCron carbonyls. In this connection theoretical studies have been performed on the simplest such compounds H2C2Fe2(CO) n (n =6, 5) for comparison with the experimentally known structure of the t-butyl derivative t-Bu2C2Fe2(CO)6 and in order to predict the decarbonylation pathways for such (alkyne)Fe2(CO)6 derivatives. These theoretical studies predict an Fe2C2 tetrahedrane structure for H2C2Fe2(CO)6 with a formal Fe bond very similar to the experimental structure for t-Bu2C2Fe2(CO)6. Decarbonylation of H2C2Fe2(CO)6 is predicted to give an H2C2Fe2(CO)5 isomer retaining the Fe2C2 tetrahedrane structure, with an Fe bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M2C2 tetrahedrane structure. The formation of formal Fe bonds appears to be avoided in even the higher energy H2C2Fe2(CO)5 structures. These include three triplet Fe2C2 tetrahedrane structures with formal Fe bonds as well as a coordinately unsaturated singlet structure, still with an Fe bond. [Copyright &y& Elsevier]

Abstract: The two cyclooctatetraene metal carbonyls that have been synthesized are the tetrahapto derivative (η4-C8H8)Fe(CO)3 and the hexahapto derivative (η6-C8H8)Cr(CO)3 using the reactions of cyclooctatetraene with Fe(CO)5 and with fac-(CH3CN)3Cr(CO)3, respectively. Related C8H8M(CO) n (M=Ti, V, Cr, Mn, Fe, Co, Ni; n =4, 3, 2, 1) species have now been investigated by density functional theory in order to explore the scope of cyclooctatetraene metal carbonyl chemistry. In this connection, the existence of octahapto (η8-C8H8)M(CO) n species is predicted as long as the central metal M does not exceed the 18-electron configuration by receiving eight electrons from the η8-C8H8 ring. Thus the lowest energy structures (η8-C8H8)Ti(CO) n (n =3, 2, 1), (η8-C8H8)M(CO) n (M=V, Cr; n =2, 1), and (η8-C8H8)Mn(CO) all have octahapto η8-C8H8 rings. An exception is (η6-C8H8)Fe(CO), with a hexahapto η6-C8H8 ring and thus only a 16-electron configuration for the iron atom. Hexahapto (η6-C8H8)M(CO) n structures are predicted for the known (η6-C8H8)Cr(CO)3 as well as the unknown (η6-C8H8)Ti(CO)4, (η6-C8H8)V(CO)3, (η6-C8H8)Mn(CO)2, and (η6-C8H8)Fe(CO)2 with 18, 18, 17, 17, and 18 electron configurations, respectively, for the central metal atoms. There are two types of tetrahapto C8H8M(CO) n complexes. In the 1,2,3,4-tetrahapto (η4-C8H8)M(CO) n complexes two adjacent Cbonds, forming a 1,3-diene unit similar to butadiene, are bonded to the metal atom. In the 1,2,5,6-tetrahapto (η2,2-C8H8)M(CO)3 derivatives two non-adjacent Cbonds of the C8H8 ring are bonded to the metal atom. The known (η4-C8H8)Fe(CO)3 is a 1,2,3,4-tetrahapto complex. The unknown isomeric 1,2,5,6-tetrahapto complex (η2,2-C8H8)Fe(CO)3 is predicted to lie ∼15kcal/mol above (η4-C8H8)Fe(CO)3. The related 1,2,5,6-tetrahapto complexes (η2,2-C8H8)Cr(CO)4, (η2,2-C8H8)Mn(CO)4, [(η2,2-C8H8)Mn(CO)3]−, (η2,2-C8H8)Co(CO)2, and (η2,2-C8H8)Ni(CO)2 are all predicted to be low-energy structures. [Copyright &y& Elsevier]

Abstract: The structures and energetics of Cp2M2(CO) (M=Mn, Cr, and Ti) have been investigated by density functional theory (DFT) for comparison with previously reported studies on Cp2V2(CO). For all four metals energetically competitive structures are found with singlet, triplet, quintet, and septet spin multiplicities. However, significant spin contamination was found for the triplet Cp2Mn2(CO) and Cp2Cr2(CO) structures. The lowest energy structures for Mn and Cr are both septet states, namely Cp2Mn2(η2-μ-CO) with a four-electron donor bridging carbonyl group and an Mnce of 2.523Å and Cp2Cr2(μ-CO) with a two-electron donor bridging carbonyl group and a Crce of 2.436Å. However, for the Ti analogue the lowest energy structure is singlet Cp2Ti2(η2-μ-CO) with a four-electron donor bridging carbonyl group and a Tice of 2.364Å. The higher energy singlet structures of Cp2M2(CO) have the very short metal–metal distances of 1.879Å (M=Mn) and 1.729Å (M=Cr) suggesting very high formal bond orders of five and six, respectively. [Copyright &y& Elsevier]

Binuclear cycloheptatrienylchromium carbonyls of the type (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η7-C7H7 rings are not found for (C7H7)2Cr2(CO)n structures having two or more carbonyl groups. This result stands in contrast to the related (CnHn)2M2(CO)n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C7H7)2Cr2(CO)n structures have bent trihapto or pentahapto C7H7 rings and Cr≡Cr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C7H7 ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C7H7–Cr bond are predicted for (C7H7)2Cr2(CO)n (n = 6, 4, 2). The global minima found for the (C7H7)2Cr2(CO)n derivatives for n = 6, 5, and 4 are (η5-C7H7)(OC)2Cr≡Cr(CO)4(η1-C7H7), (η3-C7H7)(OC)2Cr≡Cr(CO)3(η2,1- C7H7), and (η5-C7H7)2Cr2(CO)4, respectively. The global minima for (C7H7)2Cr2(CO)n (n = 3, 2) have rearranged C7H7 groups. Singlet and triplet structures with heptahapto η7-C7H7 rings are found for the dimetallocenes (η7-C7H7)2Cr2(CO) and (η7-C7H7)2Cr2, with the singlet structures being of much lower energies in both cases. [Copyright &y& Elsevier]

DFT calculations [ABSTRACT FROM AUTHOR]