Synthesis, structure, and hydride-deuteride exchange studies of CpMoH(3)(PMe(2)Ph)(2) and theoretical studies of the CpMoH(3)(PMe(3))(2) model system

The synthesis and characterization of CpMoH(3)(PMe(2)Ph)(2), 1, is described. Compound 1 is obtained from the reaction between CpMoCl(3), PMe(2)Ph, and LiAlH4, in 61% yield. Compound 1 has also been obtained from the reaction of CpMo(o-C(6)H(4)PMe(2))(PMe(2)Ph)(2) with H-2. Characterization of 1 by H-1- and P-31-NMR spectroscopies shows a high degree of fluxionality for the hydride atoms, even at low temperatures. A single-crystal X-ray structure indicates that the geometry is pseudo-octahedral, with a relative mer arrangement of the three H ligands and the two PMe(2)Ph ligands occupying relative trans positions. This is in contrast with the only other previously reported structure of a Mo(IV) trihydride, Cp*MoH3(dppe), which adopts a pseudo trigonal prismatic structure. The Mo-P (average 2.41(8); Angstrom) and Mo-H distances (average 1.64(4) Angstrom) are similar to those found in Cp*MoH3(dppe). The closest H-H distance is 1.79 Angstrom consistent with a classical hydride. The results of ab initio calculations for the CpMoH(3)(PMe(3))(2) model in different configurations agree with the experimental observations and suggest that a mechanism of hydride exchange consisting of a Bailar twist, which interconverts pseudo-octahedral mer, trans and fac geometries, is possible. The process of hydride-deuteride exchange of 1 in C6D6 is also examined.