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.© 1994, American Chemical Society. All rights reserved.