<span style="font-size:21.0pt;mso-bidi-font-size:14.0pt;color:black">Coenzyme B₁₂<span style="font-size:16.0pt;mso-bidi-font-size:9.0pt;font-family:Arial;color:black"> <span style="font-size:21.0pt;mso-bidi-font-size:14.0pt;color:black">model studies: Equilibria and kinetics of axial ligation of alkyl(aquo)cobaloximes by N donor ligands </span></span></span>

678-686Equilibria and kinetics of the reaction of trifluoroethyl(aquo)cobaloxime and iodomethyl(aquo)cobaloxime with histamine, histidine, glycine and ethyl glycine ester have been studied as a function of pH at 25°C, 1.0M KCI ionic strength by spectrophotometry technique. Comparison of equilibrium constants and rate constants indicates that the order is <i style="mso-bidi-font-style: normal">KHisn>KHiamn> KGly>KGiyest· <span style="font-size:15.0pt; mso-bidi-font-size:8.0pt;color:black">The rate of substitution of H2O varies with the <span style="font-size:15.0pt;mso-bidi-font-size: 8.0pt;font-family:Arial;color:black">pKa <span style="font-size: 15.0pt;mso-bidi-font-size:8.0pt;color:black">of the incoming ligand establishing the existence of nucleophilic participation of the ligand in the transition state. The rate constants and equilibrium constants are correlated to the hardness and softness of the ligands and Co(III) of cobaloxime. Triflouroethyl(aquo)cobaloxime forms more stable complexes than iodomethyl(aquo)cobaloxime.