Chromium−Chromium Multiple Bonding in Cr2(CO)9

Abstract

Density functional theory (DFT) is used to obtain the first structural characterization of the unsaturated dichromium carbonyl Cr2(CO)9, which is predicted to have a remarkably short metal−metal bond length of 2.31 Å (B3LYP) or 2.28 Å (BP86). This chromium−chromium distance is essentially identical to that reported experimentally for the established Cr⋮Cr triple bond in (η5-Me5C5)2Cr2(CO)4. The dissociation energy to the fragments Cr(CO)4 and Cr(CO)5 is determined to be 32 kcal/mol (B3LYP) or 43 kcal/mol (BP86). For comparison, the Cr2(CO)10 molecule and the saturated Cr2(CO)11 system have negligible dissociation energies. The minimum energy Cr2(CO)9 structure is of Cs symmetry with the two chromium atoms asymmetrically bonded to the bridging carbonyls. However, within 0.1 kcal/mol lies a C2 symmetry structure with one symmetric and two asymmetric bridging carbonyls. Furthermore, the high symmetry D3h structure analogous to Fe2(CO)9 lies only ∼1 kcal/mol higher in energy. The Cr2(CO)9 molecule is thus highly fluxional. The extremely flat potential energy surface in the region adjacent to these minima suggests that Cr2(CO)9 will be labile. The relationship between the Cr2(CO)9 molecule and the experimentally known binuclear manganese (η5-Me5C5)2Mn2(μ-CO)3 compound is explored.