Theoretical study of gas-phase reactions of Fe(CO)(5) with OH- and their relevance for the water gas shift reaction

Revision of the homogeneously Fe(CO)(5)-catalyzed water gas shift reaction in the gas phase has been performed by means of quantum chemical calculatio ns using gradient-corrected density functional theory (B3LYP) and ab initio methods at the CCSD(T) level. The classically assumed reaction path has be en scrutinized step by step, and enlarged with novel mechanistic proposals. Our calculations lend additional credit to some of the previously accepted steps in the catalytic cycle, such as the initial attack of OH- to Fe(CO)( 5) and also to the recently accepted decarboxylation of(CO)(4)FeCOOH- (via a concerted mechanism involving a four-centered transition state), as well as to the acidification of the metal, hydride (CO)(4)FeH- with water to yie ld the dihydride (CO)(4)FeH2. The present investigation also examines in te rms of energies and activation barriers the existence/participation of new intermediates tin particular, a metalloformate species, a water-hydride add uct, and a dihydrogen complex), not mentioned in prior studies. Finally, a transition-metal-containing S(N)2-type reaction is explored for the last st ages of this chemical process as a mechanistic alternative to regenerate th e starting catalyst.