The kinetics of the La3+-catalyzed methanolysis of N-phenyl-beta-lactam (2) and N-p-nitrophenyl-beta-lactam (3) as well as that of nitrocefin (1) were studied at 25 degreesC under buffered conditions. In the case of 2 and 3, the observed second-order rate constants (k(2)(obs)) for catalysis plateau at similar to(s)(s)pH 7.5-7.8, reaching values of 1 X 10(-2) and 35 x 10(-2) M-1 s(-1) respectively. Potentiometric titrations of solutions of 2 x 10(-3) M La(OTf)(3) were analyzed in terms of a dimer model (La-2(3+)(-OCH3)(n)) where the number of methoxides varies from 1 to 5. The species responsible for catalysis in the (s)(s)pH range investigated contain 1-3 methoxides, the one having the highest catalytic activity being La-2(3+)(-OCH3)(2), which comprises 80% of the total La3+ forms present at its (s)(s)pH maximum of 8.9. The catalysis afforded by the La3+ dimers at a neutral (s)(s)pH is impressive relative to the methoxide reactions: at (s)(s)pH 8.4 a 1 mM S S solution of catalyst (generated from 2 mM La(OTf)(3)) accelerated the methanolysis of 2 by similar to2 x 10(7)-fold and 3 by similar to5 x 10(5)-fold. As a function of metal ion concentration, the La3+-catalyzed methanolysis of 1 proceeds by pathways involving first one bound metal ion and then a second La3+ leading to a plateau in the k(obs) VS [La3+](total) plots at all-first-order rate constants at the plateaus, representing the spontaneous,pH values. The k(max)(obs) pseudo methanolysis of La-2(3+)(1(-)) forms, has a linear dependence on [-OCH3] (slope = 0.84 +/- 0.05 if all (s)(s)pH values are used and 1.02 +/- 0.03 if all but the two highest (s)(s)pH values are used). The speciation of bound 1 at a La3+ concentrations corresponding to that of the onset of the kinetic plateau region was approximated through potentiometric titration of the nonreactive 3,5-dinitrobenzoic acid in the presence of equiv of La(OTf)(3). A total speciation diagram for all bound forms of La-2(3+)(1(-))(-OCH3)(n), where n = 0-5, was constructed and used to determine their kinetic contributions to the overall (s)(s)pH vs k(max)(obs) plot under kinetic conditions. Two kinetically equivalent mechanisms were analyzed: methoxide attack on La-2(3+)(1(-))(-OCH3)(n), n = 0-2; unimolecular decomposition of the forms La-2(3+)(1(-))(-OCH3)(n), n = 1-3.