Intramolecular rearrangements in six-coordinate ruthenium and iron dihydrides

Molecular orbital calculations at the ab initio level are used to study pol ytopal rearrangements in H2Ru(PH3)(4) and H2Fe(CO)(4) as models of 18-elect ron, octahedral metal dihydrides It is found that, in both cases, the trans ition state for these rearrangements is a dihydrogen species. For H2Fe(CO)( 4), this is a square pyramidal complex when the Hz ligand occupies an apica l position and is rotated by 45 degrees from its original orientation. This is precisely analogous to the transition state for Fe-olefin rotation in ( olefin)Fe(CO)(4) complexes and has a very similar electronic origin. Anothe r transition state very close in energy is found wherein the basic coordina tion geometry is a trigonal bipyramid and the H-2 ligand is coordinated in the axial position. For H2Ru(PH3)(4), the former stationary point lies at a much higher energy and the latter clearly serves as the transition state f or hydride exchange. The reason for this difference is discussed along with the roles of electron correlation in the two compounds.