Modeling Fischer-Tropsch Chemistry: The Thermochemistry and Insertion Kinetics of CIRuH(CH,)

The insertions of metal-bound CH, into M-H and M 4 H 3 bonds have been proposed as the chain initiation and propagation steps, respectively, in the Fischer-Tropsch reductive polymerization of CO to form alkanes. As a model for this important elementary reaction, we have examined the properties and migratory insertion reactivity of a prototypical coordinatively saturated complex ClRuH(CH2) using ab initio methods (generalized valence bond + configuration interaction). The Ru=CH2 double bond is covalent, with D,(Ru=C) = 84.7 kcal/mol. The optimum geometry has the CH2 plane perpendicular to the ClRuH plane, with a rotational barrier of 113.6 kcal/mol. The lowest energy conformer of the singlet state of ClRuH(CH,) has an in-plane ?r bond, which facilitates the insertion of the CH2 ligand into the adjacent Ru-H bond. Using analytic gradient techniques combined with GVB-MCSCF wave functions to find the minimum energy pathway, we find that the insertion proceeds with a moderate barrier (11.5 kcal/mol) and is exothermic by 7.1 kcal/mol. From a thermodynamic cycle designed to probe basis set and electron correlation deficiencies, we estimate an actual barrier to insertion of 10.9 f 1.7 kcal/mol and an exothermicity of 10.5 f 1.0 kcal/mol.