The Quest for Metal-Metal Quadruple and Quintuple Bonds in Metal Carbonyl Derivatives: Nb2(CO)9 and Nb2(CO)8

The synthesis by Power and co-workers of the first metal-metal quintuple bond (Science 2005, 310, 844) is a landmark in inorganic chemistry. The 18-electron rule suggests that Nb-2(CO)(9) and Nb-2(CO)(8) are candidates for binary metal carbonyls containing metal-metal quadruple and quintuple bonds, respectively. Density functional theory (MPW1PW91 and BP86) indeed predicts structures having very short Nb-Nb distances of similar to 2.5 angstrom for Nb-2(CO)(9) and similar to 2.4 angstrom for Nb-2(CO)(8) as well as relatively large Nb-Nb Wiberg bond indices supporting these high formal Nb-Nb bond orders. However, analysis of the frontier molecular orbitals of these unbridged structures suggests formal Nb equivalent to Nb triple bonds and 16-electron metal configurations. This contrasts with an analysis of the frontier orbitals in a model chromium(I) alkyl linear CH3CrCrCH3, which confirms the generally accepted presence of chromium-chromium quintuple bonds in such molecules. The presence of Nb equivalent to Nb triple bonds rather than quadruple or quintuple bonds in the Nb-2(CO)(n) (n = 9, 8) structures frees up d(xy) and d(x(2)-y(2)) orbitals for d pi -> p pi* back-bonding to the carbonyl groups. The lowest energy Nb-2(CO)(n) structures (n = 9, 8) are not these unbridged structures but structures having bridging carbonyl groups of various types and formal Nb-Nb orders no higher than three. Thus, the two lowest energy Nb-2(CO)(9) structures have Nb equivalent to Nb triple bond distances of similar to 2.8 angstrom and three semibridging carbonyl groups, leading to a 16-electron configuration rather than an 18-electron configuration for one of the niobium atoms. The lowest energy structure of the highly unsaturated Nb-2(CO)(8) is unusual since it has a formal single Nb-Nb bond of length similar to 3.1 angstrom and two four-electron donor eta(2)-mu-CO groups, thereby giving each niobium atom only a 16-electron configuration.