Kinetics of Anion Exchange between Methyl Mercuric Complexes

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Abstract
In the protonmagnetic resonance spectrum of CH3HgBr or of CH3HgI, the satellites produced by 199Hg–H spin coupling are broader than in other methyl mercuric compounds. This broadening, formerly attributed to alkyl exchange, is better explained by fast relaxation of the 199Hgnuclear spin. The large quadrupole moments of bromine and iodine are probably responsible for the 199Hgrelaxation. Anion exchange of various pairs of methyl mercuric compounds (studied by the line‐shape method of Gutowsky and Holm) was too fast to measure unless cyanide was one of the anions. Rate constants and heats and entropies of activation for exchange of cyanide with hydroxide, chloride, sulfite, and thiosulfate were calculated assuming an anionic attack mechanism, e.g., CH3HgCN+X−=CH3HgX+CN−. For exchange of CH3HgCN with the hydroxide complex below pH 9 or with the mercaptoacetate complex, anionic attack seems inadequate to account for the rates, so direct reaction between CH3HgCN and the other methyl mercuric compound is postulated to occur at the high methyl mercuric concentrations used in NMR.