Abstract
Possible mechanisms of cleavage of "saturated" carbon to metal bonds, together with evidence for some of the mechanisms, have been reviewed. The reactions between mercuric iodide and tetraa-Ikyltins in "96%" methanol solvent were found to proceed by a combination of an irreversible reaction (1) and reversible reaction (2), [equations] (where (R') is the alkyl group cleaved, and (R') may equal (R)). This combination provides a novel situation in reaction kinetics. The rate equation (for the formation of R'HgI) has been integrated, both directly and by a numerical method. The equilibrium constant (K) for reaction (2) has been measured for (R) = Me, Et, n-Pr, iso-Pr, n-Bu and iso-Bu. The second---order rate constant reaction (1). has been calculated for reactions with (R')=(R)=Me, Et, n-Pr, n-Bu, iso-Bu and (R')=Me,(R)=n-Bu, at both 25 and 40 C, and (R')=(R)= iso-Pr at 40 C. The enthalpy, entropy and free energy of activation have been calculated for the above reactions. The relative rate sequence for the reactions with symmetrical tetraalkyltins (R'=R), Me >> Et > n-Pr ~ n-Bu > iso-Bu >> iso-Pr, is dominated by steric effects, by far the most important of which are the interactions between the group being cleaved (R') and the attacking reagent (Hgl[2]), and between (R') and the leaving tin atom. The rate of reaction was increased in the presence of an added, inert, salt in the reactions studied, ((R')=(R)=Et, n-Pr, n-Bu). In the cases where (R')=(R)=Et, n-Bu, it was shown that both the initial and transition states are "salted--- out"; the initial states more than the transition states. The observed increase in rate in the presence of added salt is thus due mainly to the effect of the added salt on the initial states. The mechanism of the irreversible reaction (1) is postulated to be S[E]2, involving an 'open' transition state which is more polar than the initial states.