Despite the fact that Ni(CO)4 was discovered more than a century ago, no neutral Ni2(CO) y compound has ever been synthesized in macroscopic amounts. In this study we consider a number of such compounds, including the Ni−Ni single-bonded (μ-CO)Ni2(CO)6, the NiNi double-bonded (μ-CO)2Ni2(CO)4, and the Ni⋮Ni triple-bonded (μ-CO)3Ni2(CO)2. The predicted central bond distances are 2.73 (Ni−Ni), 2.56 (NiNi), and 2.20 Å (Ni⋮Ni). The latter compound is predicted to be bound by 34 kcal/mol with respect to Ni(CO)4 + NiCO. Prospects for the synthesis of related dinickel compounds are discussed in some detail.

Homoleptic binary cobalt carbonyls with multiple cobalt−cobalt bonds have been examined theoretically using established levels of density functional methodology. These species include 19 structures ranging from the experimentally well characterized dibridged (CO)3Co(CO)2Co(CO)3 to the proposed monobridged (CO)2Co(CO)Co(CO)2 structure with a formal quadruple bond. Consistent with experiment, three energetically low-lying equilibrium structures of Co2(CO)8 were found, of C 2 v (dibridged), D 3 d (unbridged), and D 2 d (unbridged) symmetry. For Co2(CO)8, the BP86 method predicts the dibridged structure to lie 3.7 kcal/mol below the D 2 d structure and 6.3 kcal/mol below the D 3 d structure. The D 2 d and D 3 d structures thus have the opposite energetic ordering of that deduced from experiment by Sweany and Brown. A satisfactory harmony between theoretical and experimental vibrational frequencies and IR intensities is found, although the D 2 d and D 3 d structures are essentially indistinguishable in this regard. For Co2(CO)7 the unbridged asymmetric structure suggested by Sweany and Brown is confirmed with the BP86 method, and with perhaps one exception the vibrational features agree well for theory and experiment. For Co2(CO)6 only one vibrational feature has been assigned from experiment, but this band (2011 cm-1) fits very well with BP86 predictions for the monobridged D 2 d symmetry structure with a formal Co⋮Co triple bond. For the Co2(CO)5 molecule, for which no experimental results exist, the most interesting structure is the monobridged closed-shell singlet with a very short (2.17 Å) cobalt−cobalt bond, to which we assign a formal bond order of four. Potential energy distributions have been analyzed to identify the principal vibrations with cobalt−cobalt stretching contributions. The condensed phase Raman analysis by Onaka and Shriver of the Co−Co stretches for the three known isomers of Co2(CO)8 is remarkably consistent with the present predictions for the gas-phase species. Prospects for the synthesis of these and related dicobalt compounds are discussed.

The thermodynamic stability of dichromium carbonyls is investigated with density functional theory (DFT). The results demonstrate why [(μ-H)Cr2(CO)10]- has been observed while the Cr2(CO)11 and (μ-H)2Cr2(CO)9 structures remain unknown. The related structure [(μ-H)2Cr2(CO)8]2- is predicted to be stable with respect to its fragments and isolable. Homoleptic chromium carbonyl structures of the formula Cr2(CO)11 appear to be thermodynamically unstable with respect to dissociation to the fragments Cr(CO)6 and Cr(CO)5 and only slightly metastable with respect to the transition state leading to these dissociated fragments. The potential energy surface in the region adjacent to these minima appears to be very flat. In contrast, both the BP86 and B3LYP functionals predict the known [(μ-H)Cr2(CO)10]- to have significant stability with respect to the fragments Cr(CO)5 + [Cr(CO)5H]-. For the B3LYP functional, the dissociation energy is 41 kcal/mol, while for BP86 it is 43 kcal/mol. A notable structural difference for [(μ-H)Cr2(CO)10]- between the two theoretical methods is that the BP86 functional predicts the Cr−H−Cr angle to be 147° while the B3LYP functional predicts a linear geometry (180°). Experimental structures of [(μ-H)Cr2(CO)10]- determined by neutron diffraction and by X-ray crystallography display a remarkably similar ambiguity in the Cr−H−Cr angle. Certain other differences between the B3LYP and BP86 functionals are observed in the predicted geometries, numbers of imaginary vibrational frequencies, and particular energy differences. Several subtle comparisons suggest that the BP86 method is preferable to B3LYP for this particular class of compounds.