The coordination of N-donor ligands to MTO (methyltrioxorhenium) is governed by both electronic and steric effects. For example, the binding constant of pyridine to MTO is 196.6 L mol(-1), whereas that of the better donor 4-picoline is 732 L mol(-1) and that of the sterically encumbered 2,6-di-tert-butyl-4-methylpyridine is <1 L mol(-1). Equilibrium constants have been evaluated for this reaction, MTO + L = MTO.L, where L comprises mono- and bidentate N-donor ligands. The values of log K for monodentate ligands range from <0 for 2-substituted pyridines to 3.3 for 1-butylimidazole and for bidentate ligands from 2.2 for 2,2'-bipyridine to 5.27 for 4,7-dimethyl-1,10-phenanthroline at 25 degreesC in chloroform. A successful correlation of log K with pK(a) of L was realized except in the case of 2-substituted ligands, where steric effects make K smaller than expected from the proton basicity of L.