The Yb3+-catalyzed cleavage of a series of eight methyl aryl phosphates (2a-h) where the aryl groups all contain an ortho-nnethoxycarbonyl group was studied in acidic methanol from 1.34 <= (s)(s)pH <= 3.34 at 25 degrees C. All substrates show saturation binding of the metal ion that is analyzed to provide a conditional binding constant (K-b) for a 1:1 substrate/Yb3+ complex and catalytic rate constant (k(cat)) that varies between about 2 x 10(-3) and 50 x 10(-3) s(-1) over the range of substrates. Detailed analysis indicates that at very low concentration of Yb3+, 3 equiv of substrate are bound, and with increasing [Yb3+], the binding changes to a 1:1 complex which decomposes by a pathway independent of (s)(s)pH over the range investigated. Control studies show that substrates without the o-methoxycarbonyl group still bind to the Yb3+ with approximately the same strength as do the o-methoxycarbonyl containing substrates but have no observable reaction when bound. A Jaffe plot of the Kat vs substituent sigma-values indicates that, during the catalyzed reactions of 2a-h, the phenoxy-O and C(O)OCH3 groups accommodate negative and positive charge respectively, the rho(phosphate) and rho(C(O)OMe) values being (1.84 +/- 011) and (-0.85 +/- 0.14). For all these substrates, the final reaction products are dimethyl phosphate and the Yb3+ complex of the phenoxide. A study of the binding of the parent phenols to Yb3+ indicates that log(K-bind) = (0.84 +/- 0.06)(s)(s)pK(a) + (3.4 +/-0.9), r(2) = 0.9664 for phenols containing the o-methoxycarbonyl group; for those lacking that substituent log(K-bind) = (0.96 +/-0.04)(s)(s)pK(a) - (1.73 +/- 0.4), (r(2) = 0.99). For the catalyzed reaction the beta(Ig) = -0.48, while the beta(eq) = -0.95, leading to a Leffler parameter of alpha = 0.51. A mechanism is presented for the catalyzed reaction which is highly dissociative, having a transition state where the Yb3+ translocates during the cleavage reaction to assist the leaving group's departure with weak nucleophilic assistance by the solvent methanol. A comparison of the catalyzed rate of reaction with a computed rate of reaction attributable to solvent alone indicates that Yb3+ provides leaving group assistance on the order of 10(12)-fold, stabilizing the transition state for cleavage by some 16 kcal/mol.