THE ROLE OF ATE COMPLEXES IN HALOGEN(METALLOID)-METAL EXCHANGE-REACTIONS - A THEORETICAL-STUDY

Correlated electronic structure calculations predict that [(CH3)(n+1)X ](-) methyl ate anions, where X is an element of the main groups 14, 1 5, 16, or 17 up to Bi, possess widely varying stabilities that are gov erned by the electronegativities of their central atoms X, These stabi lities correlate well with the propensities of the elements in questio n to undergo exchange with Lithium and magnesium halide, except in the cases where steric hindrance in the transition states of the exchange reactions is important. These findings are nicely confirmed by calcul ations of the transition states [(CH3)(2)XLi]double dagger (X = Cl, Br , I) and [(CH3)(3)SeLi]double dagger of the corresponding degenerate e xchange reactions CH,X (X = Cl, Br, I) + CH3Li and (CH3)(2)Se + CH3Li, respectively. The computed relative stabilities of the mixed [R-I-CH3 ](-) ate anions of iodine (where R = phenyl, ethynyl, vinyl, ethyl, or cyclopropyl) are in excellent agreement with the experimentally obser ved equilibria of the corresponding lithium-iodine exchange reactions. The recent experimental observation of a highly stable cx-iodine-subs tituted iodine ate complex as an intermediate in an iodine-magnesium b romide exchange reaction is also corroborated by our studies. Thus, th e present calculations provide strong evidence for ate complexes being key intermediates in halogen(metalloid)-lithium(magnesium halide) exc hange reactions.