Thermodynamic studies of hydride transfer for a series of niobium and tantalum compounds

Reported herein are the first thermodynamic hydricities (DeltaG(M+)) for a collection of niobocene and tantalocene compounds (1-13) of general formula s (C5R4R')(2)M(H)(L) (R = H, Me; R' = H, Me, SiMe3; L = CO, PEt3, (H)(2), P hCCPh, CH2=O, =O) and (C5H4SiMe3)(2)Nb(Cl)(L') (L' = O=C=CEtPh, O=C=CEt2). The studies were carried out in acetonitrile solution, and the compounds sh ow hydride transfer free energies ranging from 74 to 95 kcal/mol, as determ ined by bracketing studies involving a known series of hydride accepters. T he data indicate that the free energies of hydride transfer are heavily inf luenced by coligands, but less so by 4d vs 5d effects in isostructural niob ium or tantalum compounds.