TRANSITION STRUCTURES OF METHANE ELIMINATION IN PENTAMETHYLNIOBIUM AND PENTAMETHYLTANTALUM

Intramolecular methane elimination through a-hydrogen abstraction in M (CH3)(5) (M = Nb, Ta) has been studied in detail with ab initio quantu m mechanics calculations. Geometry optimizations were performed with t he 3-21G and HW3 (equivalent to the 6-31G) basis sets. The energies w ere further evaluated with the MP2/HW3 calculations. Although the M(CH 3)(5) reactants significantly favor square-planar structures, the most favorable transition structure for unimolecular methane elimination i s close to a trigonal- bipyramidal geometry. Hydrogen abstraction is c oncerted, although there is significant M-H interaction in the transit ion structure. The calculated activation free energy is 35.3 and 37.3 kcal/mol for Nb(CH3)(5) and Ta(CH3)(5), respectively, at the best leve l of calculation. A dimeric mechanism through intermolecular hydrogen abstraction is found to be much lower in activation free energy than t he unimolecular mechanism. The stabilization for the dimeric transitio n structure is mainly due to the formation of an M-CH2-M bridge. Intra molecular methane eliminations in (CH3)(4)M-CH2-M(CH3)(4) were also st udied.