Single-crystal X-ray and neutron diffraction structure determination and inelastic neutron scattering study of the dihydrogen complex trans-[Ru(H-2)(H)(dppe)(2)] [BPh4]

The structure of the complex trans-[Ru(eta(2)-H-2)(H)(dppe)(2)][BPh4] (1), dppe=PPh2CH2CH2PPh2, has been determined by single-crystal X-ray diffraction at 123 K and neutron diffraction at 12 K, The core of the complex has a distorted octahedral geometry about ruthenium with the dihydrogen ligand trans to hydride and eclipsing a trans-P-Ru-P asis that is bent away from the hydrogens with a P-Ru-P angle of 167.9(4)degrees. The crystallographically determined H-H distance is 0.83(8) (X-ray) or 0.82(3) (neutron) Angstrom. The latter value, when corrected for the shortening caused by the torsional libration of the H-2 ligand, increases to about 0.94 Angstrom. The long Ru-(H-2) distance of 1.81(2) Angstrom (neutron), compared to the terminal hydride to ruthenium distance of 1.64(2) Angstrom (neutron), is consistent with the lability of the dihydrogen ligand, which is partially lost from the crystal by treatment with vacuum. The analogous iron complex trans-[Fe(eta(2)-H-2)(H)(dppe)(2)][BPh4] (2) has similar features except that the Fe-H(H-2) distances are much shorter and the H-2 ligand is correspondingly less labile. An inelastic neutron scattering study of the powder of 1 at 5 K reveals two broad inelastic peaks flanking the elastic peak. With the assumption that the dihydrogen librates in a double-minimum potential, the barrier to dihydrogen reorientation is calculated to be 1.0 to 1.4 kcal mol(-1), depending upon which of the H-H distances is used, This barrier is less than that for the iron analog, determined for its BF4 salt, therefore suggesting that there may be less d(pi) --> sigma* backbonding in 1 than 2.