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

Abstract

The synthesis by Power and co-workers of the first metal-metal quintuple bond (Science2005, 310, 844) is a landmark in inorganic chemistry. The 18-electron rule suggests that Nb2(CO)9 and Nb2(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 ∼2.5 Å for Nb2(CO)9 and ∼2.4 Å for Nb2(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≡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≡Nb triple bonds rather than quadruple or quintuple bonds in the Nb2(CO)n (n = 9, 8) structures frees up d(xy) and d(x(2)-y(2)) orbitals for dπ→pπ* back-bonding to the carbonyl groups. The lowest energy Nb2(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 Nb2(CO)9 structures have Nb≡Nb triple bond distances of ∼2.8 Å 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 Nb2(CO)8 is unusual since it has a formal single Nb-Nb bond of length ∼3.1 Å and two four-electron donor η(2)-μ-CO groups, thereby giving each niobium atom only a 16-electron configuration.