Bis(η^5-Pentamethylcyclopentadienyl) Complexes of Niobium and Tantalum

Bent metallocene derivatives having η^5-C_5Me_5 ligands (“permethylmetallocene” derivatives) offer advantages over their η^5-C_5H_5 analogs in that they generally display greater thermal stability and are less prone to dimerize through single-atom bridges. These properties have allowed the development of a rich chemistry of group 3 and 4 d-block and f-block transition metal compounds with pentamethylcyclopentadienyl ancillary ligands. Although permethylniobocene and permethyltantalocene derivatives display reactivities similar to the parent niobocene and tantalocene analogs, their increased electron density at niobium and tantalum, their greater steric crowding, and their higher thermal stabilities offer some advantages in probing fundamental transformations such as α- and β-migratory insertion and elimination processes. Convenient synthetic entry points to these compounds are (η^5-C_5Me_5)_2NbCl_2 and (η^5-C_5Me_5)_2TaCl_2. These compounds can be reduced under dihydrogen to yield (η^5-C_5Me_5)_2MH_3 (M═Nb, Ta), which catalyzes the exchange of molecular hydrogen with benzene-d^6