作者:Douglas G. Kelley、Adam Marchaj、Andreja Bakac、James H. Espenson
DOI:10.1021/ja00020a020
日期:1991.9
The reactions of Ni(1,4,8,11-tetraazacyclotetradecane)2+ with alkyl radicals were studied. Data were collected for the R,R,S,S (or beta) and R,R,R,R (or alpha) isomers. The reactions form sigma-bonded organometallic cations, as in the equation NiL2+ + R. + H2O reversible RNiL(H2O)2+. The values of the rate constants for the forward (colligation) and reverse (homolysis) reactions were measured separately by laser flash photolysis techniques with a kinetic probe. The values of k(col) lie in the order CH3 >> primary >> secondary, and just the reverse order applies to k(hom). Thus the equilibrium constants for formation of RNiL(H2O)2+, given by k(col)/k(hom), are most favorable for methyl and least favorable for secondary alkyls. These nickel-alkyl bonds are much weaker than those in corresponding cobalt and chromium complexes. For that reason, values of k(col) vary widely with the nature of R.. Values of k(col) are considerably higher for alpha-Ni(cyclam)2+ than for the beta-isomer, and the reverse order applies to k(hom). Also, the alpha-isomer has the highest equilibrium constant for the formation equilibrium. The kinetic and thermodynamic trends can be rationalized by structural effects.