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Azulene is reported to react with Mn2(CO)10to give trans-C10H8Mn2(CO)6, which has been shown by X-ray crystallography to have a bis(pentahapto) structure with no metal-metal bond. This structure is found by density functional theory to be the lowest energy C10H8Mn2(CO)6structure. However, a corresponding bis(pentahapto) cis-C10H8Mn2(CO)6structure, also without an MnMn bond, lies within ∼1 kcal mol−1of this global minimum. The lowest energy C10H8Mn2(CO)5structure is singlet cis-η5,η5-C10H8Mn2(CO)5with an Mn→Mn dative bond from the Mn(CO)3group to the Mn(CO)2group. However, a singlet cis-η6,η4-C10H8Mn2(CO)5structure with a normal Mn–Mn single bond lies within ∼6 kcal mol−1of this global minimum. The lowest energy structures of the more highly unsaturated C10H8Mn2(CO)n(n= 4, 3, 2) systems all have cisgeometries and manganese-manganese bonds of various orders. The corresponding global minima are triplet cis-η5,η3-C10H8Mn2(CO)3(η2-μ-CO) for the tetracarbonyl with a four-electron donor bridging carbonyl group, singlet cis-η5,η5-C10H8Mn2(CO)3for the tricarbonyl, and triplet cis-η6,η4-C10H8Mn2(CO)(η2-μ-CO) for the dicarbonyl. [ABSTRACT FROM AUTHOR]

The iron carbonyl nitrosyls Fe2(NO)2(CO)n (n = 7, 6, 5, 4, 3) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods, for comparison of their predicted structures with those of isoelectronic cobalt carbonyl derivatives. The lowest energy structures for Fe2(NO)2(CO)7 and Fe2(NO)2(CO)6 have two NO bridges, and the lowest energy structure for Fe2(NO)2(CO)5 has a single NO bridge with metal-metal distances (BP86) of 3.161, 2.598, and 2.426 Å, respectively, corresponding to the formal metal-metal bond orders of zero, one, and two, respectively, required for the favored 18-electron configuration for the iron atoms. The heptacarbonyl Fe2(NO)2(CO)7 is thermodynamically unstable with respect to CO loss to give Fe2(NO)2(CO)6. The favored structures for the more highly unsaturated Fe2(NO)2(CO)4 and Fe2(NO)2(CO)3 also have bridging NO groups but avoid iron-iron bond orders higher than two by formal donation of five electrons from bridging NO groups with relatively short Fe-O distances. The lowest energy structures of the unsaturated Fe2(NO)2(CO)n derivatives (n = 5, 4, 3) are significantly different from the isoelectronic cobalt carbonyls Co2(CO),n+2 owing to the tendency for Fe2(NO)2(CO)n to form structures with bridging NO groups and metal-metal formal bond orders no higher than two. [ABSTRACT FROM AUTHOR]

The binuclear (cyclobutadiene)cobalt carbonyls (4-C4H4)2Co2(CO)n(n1−4) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The singlet doubly bridged and unbridged isomers of (4-C4H4)2Co2(CO)4are nearly degenerate, suggesting a highly fluxional system similar to the experimentally known and closely related (5-C5H5)2Fe2(CO)4and (4-diene)2Co2(CO)4systems. The global minimum of (C4H4)2Co2(CO)3is the doubly bridged singlet isomer (4-C4H4)2Co2(CO)(-CO)2, in contrast to the isoelectronic (C5H5)2Fe2(CO)3, where the global minimum is the triply bridged triplet isomer (5-C5H5)2Fe2(-CO)3, analogous to dioxygen. A related triply bridged triplet isomer of (C4H4)2Co2(CO)3is also found, but at 9.6 kcal/mol (B3LYP) or 5.1 kcal/mol (BP86) above the global minimum. The (C4H4)2Co2(CO)2system behaves analogously to the corresponding (C5H5)2Fe2(CO)2system in that its lowest energy structure is the doubly bridged isomer (4-C4H4)2Co2(-CO)2with a short cobalt−cobalt distance suggestive of the CoCo triple bond required to give both cobalt atoms the favored 18-electron configuration. Both axial and perpendicular metallocene structures are found for (C4H4)2Co2(CO), including a low-energy (4-C4H4)2Co2(2--CO) structure with a four-electron-donor bridging CO group and a cobalt−cobalt distance suggestive of a CoCo triple bond. [ABSTRACT FROM AUTHOR]

The manganese carbonyl nitrosyls Mn(NO)(CO)4, Mn2(NO)2(CO)n (n = 7, 6, 5, 4), and Mn3(NO)3(CO)9 have been studied by density functional theory (DFT) using the B3LYP and BP86 methods for comparison of their predicted structures with those of isoelectronic iron carbonyl derivatives. DFT predicts a trigonal bipyramidal structure for Mn(NO)(CO)4 with an equatorial NO group very close to the experimental structure. The predicted lowest energy structure for Mn2(NO)2(CO)7 has two bridging NO groups in contrast to the known structure of the isoelectronic Fe2(CO)9, which has three bridging CO groups. The structures for the unsaturated binuclear Mn2(NO)2(CO)n (n = 6, 5, 4) derivatives are similar to those of the corresponding binuclear iron carbonyls Fe2(CO)n+2 derivatives but always with a preference of bridging NO groups over bridging CO groups. The trinuclear Mn3(NO)3(CO)9 is predicted to have a structure analogous to the known structure for Fe3(CO)12 but with two bridging NO groups rather than two bridging CO groups across one of the metal-metal edges of the M3 triangle. The dark red solid photolysis product of Mn(NO)(CO)4 characterized by its ν(CO) and ν(NO) frequencies ~45 years ago is suggested by these DFT studies not to be the originally assumed Mn2(NO)2(CO)7 analogous to Fe2(CO)9. Instead, this photolysis product appears to be Mn2(NO)2(CO)5 with a MnMn formal triple bond analogous to (η-C5H5)2V2(CO)5 obtained from the photolysis of (η5-C5H5)V(CO)4. [ABSTRACT FROM AUTHOR]

The binuclear cyclopentadienylmetal nitrosyls and carbonyls Cp2M2(AO)n (A = N, M = Fe, Co, Ni; A = C, M = Ni; n = 2, 1) are studied by density functional theory using the B3LYP and BP86 functionals. In general, structures with bridging AO ligands are energetically preferred over those with terminal AO ligands. Thus, the global minima for Cp2M2(AO)2 are all found to have closely related axial dimetallocene structures with two symmetrically bridging AO ligands but variable planarity of the central M(µ-AO)2M units. Similarly, the single AO ligands in the global minima for Cp2M2(AO) are found to bridge symmetrically the pair of metal atoms. However, structures with terminal AO groups and a single bridging Cp ligand are also found at accessible energies for CpM2(NO)(µ-Cp) (M = Fe and Co) and CpNi2(CO)(µ-Cp). The metal-metal bond distances in Cp2M2(AO)n derivatives correlate reasonably well with the requirements of the 18-electron rule. In this connection, the unusual dimer Cp2Ni2(µ-NO)2 has a Ni-Ni bond distance suggestive of a single bond and geometry suggesting one one-electron donor bridging NO group and one three-electron donor bridging NO group. However, dissociation of Cp2Ni2(µ-NO)2 into the well-known stable monomer CpNiNO is highly favored energetically. [ABSTRACT FROM AUTHOR]

The binuclear cyclopentadienyliron carbonyls Cp2Fe2(CO)n (n = 4, 3, 2, 1; Cp = η5·C5H5) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The trans- and cis-Cp2Fe2(CO)2(μ-CO)2 isomers of Cp2Fe2(CO)4 known experimentally are predicted by DFT methods to be genuine minima with no significant imaginary vibrational frequencies. The energies of these two Cp2Fe2(CO)2(μ-CO)2 structures are very similar, consistent with the experimental observation of an equilibrium between these isomers in solution. An intermediate between the interconversion of the trans- and cis-Cp2Fe2(CO)2(μ-CO)2 dibridged isomers of Cp2Fe2(CO)4 can be the trans unbridged isomer of Cp2Fe2(CO)4 calculated to be 2.3 kcal/mol (B3LYP) or 9.1 kcal/mol (BP86) above the global minimum trans-Cp2Fe2(CO)2(μ-CO)2. For the unsaturated Cp2Fe2(CO)3, the known triplet isomer Cp2Fe2(μ-CO)3 with an Fe=Fe double bond similar to the OO double bond in O2 is found to be the global minimum. The lowest-energy structure for the even more unsaturated Cp2Fe2(CO)2 is a dibridged structure Cp2Fe2(μ-CO)2, with a short Fe-Fe distance suggestive of the FeFe triple bond required to give both Fe atoms the favored 18-electron configuration. Singlet and triplet unbridged structures for Cp2Fe2(CO)2 were also found but at energies considerably higher (20–50 kcal/mol) than that of the global minimum Cp2Fe2(μ-CO)2. The lowest-energy structure for Cp2Fe2(CO) is the triplet unsymmetrically bridged structure Cp2Fe2(μ-CO), with a short Fe-Fe distance (∼2.1 Å) suggestive of the σ + 2π + ²/2δ Fe(Multiple line equation(s) cannot be represented in ASCII text)Fe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration. [ABSTRACT FROM AUTHOR]

The binuclear cyclopentadienylcobalt carbonyls Cp2Co2(CO)n (n = 3, 2, 1; Op = η5-C5H5) are studied by density functional theory using the B3LYP and 6P86 functionals. The experimentally known monobridged isomer Cp2Co2(CO)2(μ-CO) and the tribridged isomer Cp2Co2(μ-CO)3 of Cp2Co2(CO)3 with formal Co-Co single bonds are found to be similar in energy, with the precise relative energies of the two isomers depending on the functional chosen. For Cp2Co2(CO)2, the experimentally known coaxial isomer Cp2Co2(μ-CO)2 with two bridging CO groups and a formal Co=Co double bond (2.360 Å by B3LYP or 2.346 Å by BP86) is found to lie 38.2 (B3LYP) or 34.9 kcal/mol (BP86) below a perpendicular isomer ⊥-Cp2Co2(CO)2. Similarly, for Cp2Co2(CO), the coaxial isomer Cp2Co2(μ-CO) with one bridging CO group and a formal Co≡Co triple bond (2.021 Å by B3LYP or 2.050 Å by BP86) is found to lie 9.36 (B3LVP) or 9.62 kcal/mol (BP86) below the corresponding perpendicular isomer ⊥-Cp2Co2(CO). This coaxial isomer Cp2Co2(μ-CO) is a possible intermediate in the known pyrolysis of the trimer (η5-C5H5)3Co3(μ-CO)3 to give the tetranuclear complex (η5-C5H5)4Co4(μ3-CO)2. These optimized Cp2Co2(CO)n (n = 3, 2, 1) structures can be compared with the corresponding Fe2(CO)6+n structures since the CpCo and Fe(CO)3 groups are isolobal. In general, the metal-metal bonds are 0.09-0.22 Å shorter for the Cp2Co2(CO)n (n = 3, 2, 1) complexes than for the corresponding Fe2(CO)6+n complexes. For Fe2(CO)9, the experimentally well-known Fe2(CO)6(μ-CO)3 isomer is shown to be very close in energy to the unknown Fe2(CO)8(μ-CO) isomer, with the precise relative energies depending on the basis set used. [ABSTRACT FROM AUTHOR]