Cobalt−Cobalt Multiple Bonds in Homoleptic Carbonyls? Co₂(CO)_x (x = 5−8) Structures, Energetics, and Vibrational Spectra

Abstract

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.

Structure, Spectroscopy and Photochemistry of the [M(η5-C5H5)(CO)2]2 Complexes (M=Fe, Ru)

“…A similar effect was obtained for [Co 2 (CO) 8 ] [27]. It is caused by various estimations of the relative metal-metal bond and metal-carbonyl bond energies in both methods.…”

Heterodinuclear Transition-Metal Complexes with Multiple Metal–Metal Bonds

“…While it is debatable if the metal ± metal interactions in unsaturated dinuclear organometallic compounds (i.e. those with 32 or fewer valence electrons) should be viewed as multiple metal ± metal bonds, [16,84] such complexes manifest structures and reactivities which are related to those of dinuclear complexes with Werner-type ligands.…”

Solid State Applications of QTAIM and the Source Function – Molecular Crystals, Surfaces, Host–Guest Systems and Molecular Complexes

“…In the Co 2 (CO) x (x ¼ 8-5) series [84], source contributions from the Co atoms increase and become positive with decreasing net positive charge on the Co atom and with increasing formal CoaCo bond order, from one to four [69]. Trends of source contributions from Co atoms parallel the corresponding trends of d(CoaCo) delocalization indices.…”