THEORETICAL-STUDIES OF ORGANOMETALLIC COMPOUNDS .7. STRUCTURES AND BOND-ENERGIES OF THE TRANSITION-METAL HEXACARBONYLS M(CO)(6) (M=CR, MO, W) - A THEORETICAL-STUDY

The geometries of the hexacarbonyls and pentacarbonyls of chromium, mo lybdenum, and tungsten are optimized at the Hartree-Fock and MP2 level s of theory using effective core potentials for the metal atoms. The M -CO bond lengths of Mo(CO)(6) and W(CO)(6) predicted at the MP2 level using moderate valence basis sets are in excellent agreement with expe rimental values. The Cr-CO bond length in Cr(CO)(6) calculated at MP2 is too short. The total bond energies of the metal hexacarbonyls calcu lated at the CCSD(T) level of theory are slightly lower than the exper imentally derived values. The first dissociation energies calculated a t CCSD(T) using MP2-optimized geometries for M(CO)(6) and M(CO)(5) are in very good agreement with experimental results for Mo(CO)(6) and W( CO)(6) from gas-phase laser pyrolysis. The calculated first dissociati on energy at CCSD(T) for Cr(CO)(6) using the MP2-optimized geometries for Cr(CO)(6) and Cr(CO)(5) is too high. The theoretical and experimen tal results suggest the following first dissociation energies Delta H- 298 for the M(CO)(6) compounds: Cr(CO)(6) = 37 +/- 2 kcal/mol; Mo(CO)( 6) = 40 +/- 2 kcal/mol; W = 46 +/- 2 kcal/mol. The agreement of previo usly reported theoretic;al dissociation energies using density functio nal 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)(6) and W(CO)(6) refer to a different mechanism and should not be used to estimate the metal-carb onyl bond strength.