An attractive cis-effect of hydride on neighbor ligands: experimental and theoretical studies on the structure and intramolecular rearrangements of Fe(H)2(.eta.2-H2)(PEtPh2)3

The compound of formula FeH{sub 4}(PEtPh{sub 2}){sub 3} has been established by neutron diffraction to possess the structure and linkage cis,mer-Fe(H{sub 2})(PEtPh{sub 2}){sub 3}, and thus be generally similar in structure to cis,mer-Fe(H){sub 2}(N{sub 2})(PEtPh{sub 2}){sub 3}, whose structure has been determined by X-ray diffraction. The Fe-hydride distances in Fe(H){sub 2}(PEtPh{sub 2}){sub 3} are 1.538 (7) {angstrom} (trans to H{sub 2}) and 1.514 (6) {angstrom} (trans to PEtPh{sub 2}), and the Fe-H (of H{sub 2}) distances are 1.607 (8) and 1.576 (9) {angstrom}. The H-H distance in coordinated dihydrogen is 0.821 (10) {angstrom}, but the H-H bond adopts an orientation unique among structurally characterized octahedral H{sub 2} complexes: staggered with respect to the cis Fe-P and Fe-H axes. Vibrational frequencies of the Fe(H){sub 2}(H{sub 2}) substructure have been measured by difference inelastic neutron scattering spectroscopy. Neutron scattering also reveals the low-frequency rotational tunneling splitting, allowing estimation of the height of the torsional barrier for coordinated H{sub 2} rotating about its midpoint ({approximately}1 kcal/mol).