Methyl phosphite ((CH3O)P(H)(O)(2)(-); MeOPH) and methylethyl phosphate ((CH3O)P(OCH2CH3)(O)(2)(-); MEP) are two members of a class of anionic ligands whose P-31 T-2 relaxation rates are remarkably sensitive to paramagnetic metal ions, The temperature dependence of the P-31 NMR line broadenings caused by the Mn(H2O)(6)(2+) ion and a water-soluble manganese(III) porphyrin ((MnTMPyP5+)-T-III) indicates that the extent of paramagnetic relaxation enhancement is a measure of the rate at which the anionic probes come into physical contact with the paramagnetic center (i.e., enter the inner coordination shell); that is, pi Delta nu (par) = k(assn)[M], where Delta nu (par) is the difference between the line widths of the resonance in paramagnetic and diamagnetic solutions, and k(assn) is the second-order rate constant for association of the phosphorus ligand with the metal, M. Comparison of the P-31 T-1 and T-2 relaxation enhancements shows that rapid T-2 relaxation by the metal ion is caused by scalar interaction with the electronic spin. Relaxation of the phosphorus-bound proton of MeOPH (H-1-P) by (MnTMPyP5+)-T-III displayed intermediate exchange kinetics over much of the observable temperature range. The field strength dependence of H-1-P T-2 enhancement and the independence of the P-31 T-2 support these assertions. As in the case of the P-31 T-2, the H-1-P T-2 relaxation enhancement results from scalar interaction with the electronic spin. The scalar coupling interpretation of the NMR data is supported by a pulsed EPR study of the interactions of Mn(H2O)(6)(2+) with the P-deuterated analogue of methyl phosphite, CH3OP(H-2)(O)(2)(-). The elemon to P-31 and H-2 nuclear scalar coupling constants were found to be 4.6 and 0.10 MHz, respectively. In contrast, the effects of paramagnetic ions on the methoxy and ethoxy H-1 resonances of MeOPH and MEP are weak, and the evidence suggests that relaxation of these nuclei occurs by a dipolar mechanism. The wide variation in the relaxation sensitivities of the H-1 and P-31 nuclei of MeOPH and MEP permits us to study how differences in the strengths of the interactions between an observed nucleus and a paramagnetic center affect NMR T-2 relaxations. We propose that these anion ligand probes may be used to study ligand-exchange reactivities of manganese complexes without requiring variable temperature studies. The P-31 T-2 is determined by chemical association kinetics when the following condition is met:(T-2M.P/T-2M.H)-(Delta nu (P)/Delta nu (HP) - 1) < 0.2 where T-2M.P and T-2M.H are the transverse relaxation times of the P-31 and H-1 nuclei when the probe is bound to the metal, and Delta nu (P) and Delta nu (HP) are the paramagnetic line broadenings of the P-31 and H-1-P nuclei, respectively. We assert that the ratio T-2M.P/T-2M.H can be estimated for a general metal complex using the results of EPR and NMR experiments.