Tantalum-Gold and Tantalum-Copper Trihydride Complexes [Cp'(2)TaH(3)MPPh(3)][PF(6)] (Cp' = C(5)H(4)C(CH(3))(3)). Structure Determination from (1)H T(1) Relaxation Studies.

Abstract

The reaction of Cp'(2)TaH(3) (Cp' = C(5)H(4)C(CH(3))(3)) with [MPPh(3)][PF(6)] yields the bimetallic complexes [Cp'(2)TaH(3)MPPh(3)][PF(6)] (M = Au (1),M = Cu (2)). The detailed 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 exchange between two outer hydride ligands. The exchange is faster in 1 (DeltaG()(210 K) = 9.3 kcal/mol) than in 2 (DeltaH() = 8.6 +/- 0.2 kcal/mol, DeltaS() = - 5.0 +/- 0.4 eu, DeltaG() (210 K) = 9.6 +/- 0.2 kcal/mol). In contrast, complex 2 undergoes the faster intermolecular PPh(3)/PPh(3) exchange. It has been demonstrated that T(1min), T(1), T(1sel) and T(1bis) measurements are a powerful instrument for quantitative localization of the hydride ligands in solutions of bimetallic complexes. The determined hydride-hydride and metal-hydride distances reproduce well the structural tendencies in the related niobium trihydride [{Nb(C(5)H(3)RR')(2)H(3)}(2)Au][PF(6)] (R = R' = Si(CH(3))(3)) established by the X-ray method. Tantalum-gold complex 1 showed weak exchange couplings.