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 A (B3LYP) or 2.28 A(BP86). This chromium-chromium distance is essentially identical to that reported experimentally for the established Cr≡Cr triple bond in (η~5-Me_5C_5)_2Cr_2(CO)_4. This 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_(3h) 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_5C_5)_2Mn_2(μ-CO)_3 compound is explored.
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