The methanolyses of two neutral phosphorus triesters, paraoxon (1) and fenitrothion (3), were investigated as a function of added Zn(OTf)(2) or Zn(ClO4)(2) in methanol at 25 degreesC either alone or in the presence of equimolar concentrations of the ligands phenanthroline (4), 2,9-dimethylphenanthroline (5), and 1,5,9-triazacyclododecane (6). The catalysis requires the presence of methoxide, and when studied as a function of added NaOCH3, the rate constants (k(obs)) for methanolysis of Zn2+ alone or in the presence of equimolar 4 or 5 maximize at different [-OCH3]/[Zn2+](total) ratios of 0.3, 0.5, and 1.0, respectively. Plots of k(obs) VS [Zn2+](total) either alone or in the presence of equimolar ligands 4 and 5 at the [-OCH3]/[Zn2+](total) ratios corresponding to the rate maxima are curved and show a nonlinear dependence on [Zn2+](total). In the cases of 4 and 5, this is explained as resulting from formation of a nonactive dimer, formulated as a bis-mu-methoxide-bridged form (L:Zn2+(-OCH3)(2)Zn2+:L) in equilibrium with an active monomeric form (L:Zn2+(-OCH3)). In the case of the Zn2+ :6 system, no dimeric forms are present as can be judged by the strict linearity of the plots of k(obs) VS [Zn2+](total) in the presence of equimolar 6 and -OCH3. Analysis of the potentiometric titration curves for Zn2+ alone and in the presence of the ligands allows calculation of the speciation of the various Zn2+ forms and shows that the binding to ligands 4 and 6 is very strong, while the binding to ligand 5 is weaker. Overall the best catalytic system is provided by equimolar Zn2+, 5, and (-)OCH3, which exhibits excellent turnover of the methanolysis of paraoxon when the substrate is in excess. At a concentration of 2 mM in each of these components, which sets the (s)(s)pH of the solution at 9.5, the acceleration of the methanolysis of paraoxon and fenitrothion relative to the methoxide reaction is 1.8 x 10(6)-fold and 13 x 10(6)-fold, respectively. A mechanism for the catalyzed reactions is proposed which involves a dual role for the metal ion as a Lewis acid and source of nucleophilic Zn2+-bound -OCH3.