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 d ihydrogen ligand trans to hydride and eclipsing a trans-P-Ru-P asis th at is bent away from the hydrogens with a P-Ru-P angle of 167.9(4)degr ees. The crystallographically determined H-H distance is 0.83(8) (X-ra y) or 0.82(3) (neutron) Angstrom. The latter value, when corrected for the shortening caused by the torsional libration of the H-2 ligand, i ncreases to about 0.94 Angstrom. The long Ru-(H-2) distance of 1.81(2) Angstrom (neutron), compared to the terminal hydride to ruthenium dis tance of 1.64(2) Angstrom (neutron), is consistent with the lability o f the dihydrogen ligand, which is partially lost from the crystal by t reatment 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 t he assumption that the dihydrogen librates in a double-minimum potenti al, 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.