Modeling Fischer—Tropsch Chemistry: The Thermochemistry and Insertion Kinetics of CIRuH(CH₂)

The insertions of metal-bound CH_x into M-H and M-CH₃ 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 C1Ruh(Ch₂) using ab initio methods (generalized valence bond + configuration interaction). The Ru═CH₂ double bond is covalent, with D_e(Ru═C) = 84.7 kcal/mol. The optimum geometry has the CH₂ plane perpendicular to the ClRuH plane, with a rotational barrier of ≥13.6 kcal/mol. The lowest energy conformer of the singlet state of C1Ruh(Ch₂) has an in-plane π 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 ± 1.7 kcal/mol and an exothermicity of 10.5 ± 1.0 kcal/mol.