Kinetic and Equilibrium Studies of Reactions of N‐Heterocycles with Dimeric and Monomeric Oxorhenium( V ) Complexes

Equilibrium constants have been evaluated for the reaction {MeReO(edt)} 2 + 2 L ⇄ 2 MeReO(edt)L, where edt is 1,2‐ethanedithiolate and L is any of 13 N‐donor heterocyclic ligands. The values of K range from 1.37(27)×10 −2 for pyrimidine to 1.95(6)×10 6 for imidazole at 25 °C in chloroform. A successful correlation of log K with log ( K a ) of HL + was realized except in the case of the 2‐substituted ligands 2‐picoline and quinoline, where steric effects make K smaller than expected from the proton basicity of L. The kinetics of the same reactions were studied; the rate law for the reaction in the forward direction is given by −d[{MeReO(edt)} 2 ]/d t = { k a + k b [L]}[L] × [{MeReO(edt)} 2 ]. Except for 2‐picoline and quinoline, the major pathway is provided by the term that shows the quadratic dependence on [L]. Values of log ( k b ) also correlate with log K , and therefore necessarily with log ( K a ).