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 FeH4(PEtPh2)3has been established by neutron diffraction to possess the structure and linkage cis, mer-Fe(H)2(H2)(PEtPh2)3, and thus be generally similar in structure to cis, mer-Fe(H)2(N2)(PEtPh2)3, whose structure has been determined by X-ray diffraction. The Fe-hydride distances in Fe(H)2(H2)(PEtPh2)3are 1.538 (7) Å (trans to H2) and 1.514 (6) Å (trans to PEtPh2), and the Fe-H (of H2) distances are 1.607 (8) and 1.576 (9) Å. The H-H distance in coordinated dihydrogen is 0.821 (10) Å, but the H-H bond adopts an orientation unique among structurally characterized octahedral H2complexes: staggered with respect to the cis Fe-P and Fe-H axes. Vibrational frequencies of the Fe(H)2(H2) 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 H2rotating about its midpoint (∼ 1 kcal/mol). Molecular mechanics calculations predict a ground-state structure where the H-H bond eclipses the P-Fe-P direction. Extended Hückel calculations with conventional hydrogen parameters predict a structure where the H-H bond eclipses the P-Fe-P vector. However, if the hydridic character of the hydride center is considered in the calculations, the experimental conformation is found to be the most stable one. The extended Hückel results are analyzed to reveal the importance of a stabilizing overlap between the filled Fe-H σ orbital and the empty σ⋆H-H This nascent H/H2bond formation is proposed to facilitate the hydride/H2fluxionality of Fe(H)2(H2)(PEtPh2)3, in part by avoiding an intermediate with four independent hydride ligands. Crystal data for Fe(H)2(H2)(PEtPh2)3(27 K): a = 21.527 (5) Å, b = 11.753 (5) Å,c = 31.034 (7) Å, β = 112.09 (1)°, and Z = 8 in space group C2/c. Crystal data for Fe(H)2(N2)(PEtPh2)3(118 K): a = 18.355 (7) Å, b = 12.227 (3) Å, c = 19.273 (8) Å, β = 118.48 (1)°, and Z = 4 in space group P2l/n. © 1990, American Chemical Society. All rights reserved.