A proton magnetic resonance study of trimethyl phosphate exchange on magnesium(II)

A 1H N.M.R. study of the exchange reaction Mg(tmp)62+ ↔ tmp (tmp, trimethyl phosphate) in methylene chloride and acetone diluents is reported. A complete signal shape analysis is used to derive kinetic parameters. The value for kex (193 K) in the rate law R = 6kex[Mg(tmp)62+] is 3.2�0.5 s-1, ΔH? = 70.7�3.8 kJ mol-1, and ΔS? = 135.1�12.6 J mol-1 K-1 when the Mg2+ : tmp : methylene chloride mole ratio is 1.00 : 12.1 : 34.9. Values of similar magnitude are obtained under different mole ratio and diluent conditions. The tmp exchange is considered to proceed through a dissociative mechanism involving solvent structural rearrangement in the activation process.