Yd. Wu et al., TRANSITION STRUCTURES OF METHANE ELIMINATION IN PENTAMETHYLNIOBIUM AND PENTAMETHYLTANTALUM, Journal of the American Chemical Society, 117(36), 1995, pp. 9259-9264
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.