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 Cr 2 (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 (η 5 -Me 5 C 5 ) 2 Cr 2 (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 Cr 2 (CO) 10 molecule and the saturated Cr 2 (CO) 11 system have negligible dissociation energies. The minimum energy Cr 2 (CO) 9 structure is of C s symmetry with the two chromium atoms asymmetrically bonded to the bridging carbonyls. However, within 0.1 kcal/mol lies a C 2 symmetry structure with one symmetric and two asymmetric bridging carbonyls. Furthermore, the high symmetry D 3 h structure analogous to Fe 2 (CO) 9 lies only ∼1 kcal/mol higher in energy. The Cr 2 (CO) 9 molecule is thus highly fluxional. The extremely flat potential energy surface in the region adjacent to these minima suggests that Cr 2 (CO) 9 will be labile. The relationship between the Cr 2 (CO) 9 molecule and the experimentally known binuclear manganese (η 5 -Me 5 C 5 ) 2 Mn 2 (μ-CO) 3 compound is explored.