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) with 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). Crystallographic studies of each of these compounds confirmed the transoid disposition of the phosphorus substituents with respect to the MP2 ring. Reaction of Cp2TaH3 with white phosphorus afforded the parent complex Cp2TaH[(PH)2] (5). While crystallographic characterization of 5 confirmed the formulation, it failed to reveal the orientation of the hydrogen atoms on P. Variable temperature NMR studies in conjunction with selective decoupling, 2D, NOE, and NOESY experiments were performed on compound 5. These experiments were consistent with a transoid disposition of the hydrogens atoms on P at low temperature. The marked temperature dependence of the NMR data, while suggestive of a fluxional process, is more logically explained in terms of a dramatic and unusual temperature dependence of the 31P chemical shifts. These results are discussed and their implications considered.