Tantalum-gold and tantalum-copper trihydride complexes [Cp ' 2TaH3MPPh3][PF6] (Cp '=C5H4C(CH3)(3)). Structure determination from H-1 T-1 relaxation studies

The reaction of Cp'2TaH3 (Cp' = C5H4C(CH3)(3)) with [MPPh3][PF6] yields the bimetallic complexes [Cp'2TaH3MPPh3][PF6] (M = Au (1), M = Cu (2)). The de tailed NMR study of 1 and 2 showed the structure with two bridging hydride ligands. The mutual orientation of the Cp' rings has been determined by NOE experiments. The variable-temperature NMR data showed an intramolecular ex change between two outer hydride ligands. The exchange is faster in 1 (Delt a G double dagger(210 K) = 9.3 kcal/mol) than in 2 (Delta H double dagger = 8.6 +/- 0.2 kcal/mol, Delta S double dagger = - 5.0 +/- 0.4 eu, Delta G do uble dagger (210 K) = 9.6 +/- 0.2 kcal/mol). In contrast, complex 2 undergo es the faster intermolecular PPh3/PPh3 exchange. It has been demonstrated t hat T-1min, T-1, T-1sel and 2(1bis) measurements are a powerful instrument for quantitative localization of the hydride ligands in solutions of bimeta llic complexes. The determined hydride-hydride and metal-hydride distances reproduce well the structural tendencies in the related niobium trihydride [{Nb(C5H3RR')(2)H-3}(2)Au][PF6] (R = R' = Si(CH3)(3)) established by the X- ray method. Tantalum-gold complex 1 showed weak exchange couplings.