NIOBIUM AND TANTALUM DIPHOSPHANATO COMPLEXES - SYNTHESIS, STRUCTURE, AND NMR-STUDIES OF CP2MH[(PR)(2)] (R=PH, CY, H)

The reactions of Cp2MH3 (M = Nb, Ta) or Cp'2NbH3 (Cp' = Me3SiC5H4) wit h excess primary phosphines yield the complexes Cp2NbH[(PPh)(2)] (1), Cp'2NbH[(PPh)(2)] (2), and Cp2TaH[(PR)(2)] (R = Ph, 3; R = Cy, 4), Cry stallographic studies of each of these compounds confirmed the transoi d disposition of the phosphorus substituents with respect to the MP2 r ing. Reaction of Cp2TaH3 with white phosphorus afforded the parent com plex Cp2TaH[(PH2] (5). While crystallographic characterization of 5 co nfirmed the formulation, it failed to reveal the orientation of the hy drogen atoms on P. Variable temperature NMR studies in conjunction wit h selective decoupling, 2D, NOE, and NOESY experiments were performed on compound 5. These experiments were consistent with a transoid dispo sition of the hydrogens atoms on P at low temperature. The marked temp erature dependence of the NMR data, while suggestive of a fluxional pr ocess, is more logically explained in terms of a dramatic and unusual temperature dependence of the P-31 chemical shifts. These results are discussed and their implications considered.