Kinetics and mechanism of the insertion of olefins into niobium- and tantalum-hydride bonds: a study of the competition between steric and electronic effects

The mechanism of insertion of olefins into transition metal-hydride bonds has been examined for a series of olefin hydride complexes of niobocene, permethylniobocene, tantalocene, and permethyltantalocene. The insertion rates were measured for several meta-substituted styrene complexes of permethylniobocene and found to be comparable to those found for the para-substituted analogues, indicating that the electronic effects are largely inductive in origin. An X-ray crystal structure determination for the parent styrene complex (P/sub bca/; a = 14.626 (3) A, b = 18.134 (4) A, c = 18.147 (4) A; ..cap alpha.., ..beta.., ..gamma.. = 90/sup 0/) offers an explanation for this observation: the phenyl ring is bent out of resonance with the olefin ..pi.. system by 32/sup 0/. A comparison of the rates for the entire series of ethylene compounds (where steric effects are minimal) indicates that there are significant electronic effects acting within this family (k/sub Nb/ > k/sub Cp/; Cp = (eta/sup 5/-C/sub 5/H/sub 5/), Cp* = (eta/sup 5/-C/sub 5/Me/sub 5/)). In contrast to the pentamethylcyclopentadienyl systems for which steric interactions appear to dominate, the normal cyclopentadienyl compounds are virtually insensitive to sterics, allowing for the existence of two isomeric forms of the ..cap alpha..-olefin hydride complexes (endo, olefin substituent central, and exo, olefin substituent lateral in the equatorial plane) in roughly 1:1 proportion.