Intramolecular Rearrangements in Six-Coordinate Ruthenium and Iron Dihydrides

Abstract

Molecular orbital calculations at the ab initio level are used to study polytopal rearrangements in H2Ru(PH3)4 and H2Fe(CO)4 as models of 18-electron, octahedral metal dihydrides. It is found that, in both cases, the transition state for these rearrangements is a dihydrogen species. For H2Fe(CO)4, this is a square pyramidal complex where the H2 ligand occupies an apical 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. Another transition state very close in energy is found wherein the basic coordination geometry is a trigonal bipyramid and the H2 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 for hydride exchange. The reason for this difference is discussed along with the roles of electron correlation in the two compounds.

Iron Dihydride Complexes: Synthesis, Reactivity, and Catalytic Applications

“…The proposed isomerization mechanism involved hydride migration from one tetrahedron (formed by FeL 4 ) face to another by traversing the edge connecting the two faces. Computational studies of H 2 Fe(CO) 4 , however, suggested that the intramolecular rearrangement proceeded via a 5‐coordinate Fe(0) dihydrogen complex . Complex H 3 Fe(PEtPh 2 ) 3 (SnPh 3 ) was shown to contain a tetrahedral FeP 3 Sn core with three hydride ligands bridging iron and tin (Figure ) .…”

Stereochemical Nonrigidity of Organometallic Complexes

“…321 Nevertheless, some six-coordinate dihydrides of iron and ruthenium show stereochemically nonrigid behavior. 322 For example, a low-temperature limiting spectrum is observed at −50…”

“…Hydride -dihydrogen equilbria which allows conversion to five-coordination could also rationalize the low barriers for some octahedral dihydrides. 322 SST experiments have also indicated a dihydride intermediate in rearrangements of pseudooctahedral Re(CO)(NO)L 2 (H 2 ).…”

Stereochemical Nonrigidity of Organometallic Complexes

“…321 Nevertheless, some six-coordinate dihydrides of iron and ruthenium show stereochemically nonrigid behavior. 322 For example, a low-temperature limiting spectrum is observed at −50 • C for Fe[PPh(OEt) 2 ] 4 H 2 and shows both cis and trans isomers. Hydride -dihydrogen equilbria which allows conversion to five-coordination could also rationalize the low barriers for some octahedral dihydrides.…”

“…Hydride -dihydrogen equilbria which allows conversion to five-coordination could also rationalize the low barriers for some octahedral dihydrides. 322 SST experiments have also indicated a dihydride intermediate in rearrangements of pseudooctahedral Re(CO)(NO)L 2 (H 2 ). 323 Fluxionality has also been observed in chelated octahedral complexes.…”