Structures and Bond Energies of the Transition Metal Hexacarbonyls M(CO)₆(M = Cr, Mo, W). A Theoretical Study¹

The geometries of the hexacarbonyls and pentacarbonyls of chromium, molybdenum, and tungsten are optimized at the Hartree-Fock and MP2 levels of theory using effective core potentials for the metal atoms. The M-CO bond lengths of Mo(CO)₆and W(CO)₆predicted at the MP2 level using moderate valence basis sets are in excellent agreement with experimental values. The Cr-CO bond length in Cr(CO)₆calculated at MP2 is too short. The total bond energies of the metal hexacarbonyls calculated at the CCSD(T) level of theory are slightly lower than the experimentally derived values. The first dissociation energies calculated at CCSD(T) using MP2-optimized geometries for M(CO)₆and M(CO)₅are in very good agreement with experimental results for Mo(CO)₆and W(CO)₆from gas-phase laser pyrolysis. The calculated first dissociation energy at CCSD(T) for Cr(CO)₆using the MP2-optimized geometries for Cr(CO)₆and Cr(CO)₅is too high. The theoretical and experimental results suggest the following first dissociation energies ΔH²⁹⁸for the M(CO)₆compounds: Cr(CO)₆= 37 ± 2 kcal/mol; Mo(CO)₆= 40 ± 2 kcal/mol; W = 46 ± 2 kcal/mol. The agreement of previously reported theoretical dissociation energies using density functional theory with kinetic data for the activation energy of substitution reactions showing a different order for the hexacarbonyls Mo < Cr < W is misleading. The kinetic data for Mo(CO)₆and W(CO)₆refer to a different mechanism and should not be used to estimate the metal-carbonyl bond strength.