Theoretical study of the proton transfer between water and [FeH(CO)(4)](-)in aqueous solution and relevance to the water-gas shift reaction catalyzed by iron pentacarbonyl in the condensed phase

Proton transfer between H2O and FeH(CO)(4)(-), the assumed slow step of the water-gas shift reaction catalyzed by Fe(CO)(5), has been studied with Har tree-Fock and multiconfiguration CASSCF ab initio methods. The calculations have been performed in vacuo and in water solution at 298 K, simulating th e solvent as a polarizable continuum medium. The process in solution has be en found to be less endothermic than in the gas phase by about 50 kcal/mol. The valence bond analysis, made on the CASSCF wave function along the reac tion path, explains the mechanism in terms of a single structure in which t he transferred hydrogen is simultaneously bonded to iron and oxygen with a different polarization depending on the reaction coordinate.