Computational study of the spin-forbidden H2 oxidative addition to 16-electron Fe(0) complexes

Abstract

The spin-forbidden oxidative addition of H₂ to Fe(CO)₄, Fe(PH₃)₄, Fe(dpe)₂ and Fe(dmpe)₂ [dpe = H₂PCH₂CH₂PH₂, dmpe = (CH₃)₂PCH₂CH₂P(CH₃)₂] has been investigated by density functional theory using a modified B3PW91 functional. All 16-electron fragments are found to adopt a spin triplet ground state. The H₂ addition involves a spin crossover in the reagents region of configurational space, at a significantly higher energy relative to the triplet dissociation asymptote and, for the case of Fe(CO)₄·H₂, even higher than the singlet dissociation asymptote. After crossing to the singlet surface, the addition proceeds directly to the classical cis-dihydride product. Only for the Fe(CO)₄ was it possible to locate a stable energy minimum for the non-classical tautomer (dihydrogen complex), but the energy difference between this minimum and the tautomerisation transition state inverts when taking into account the zero-point energy correction. The geometries at the crossing points indicate a “side-on” approach of the H₂ molecule to the metal for the Fe(CO)₄, Fe(CO)₂(PH₃)₂ and Fe(PH₃)₄ systems. These geometries are more reactants-like for the Fe(CO)₄ system and more product-like for the Fe(PH₃)₄ system. The crossing point geometry for the Fe(dpe)₂ system, on the other hand, is nearly C₂-symmetric. The presence of an energy barrier on going from ³FeL₄ + H₂ to the crossing point is in agreement with the slow observed rates for addition of H₂ to these unsaturated organometallic fragments.