The chemistry of vitamin B₁₂. Part 22. Steric effects in the co-ordination of amines by cobalt(III) corrinoids

The following equilibrium constants (given as log₁₀K/dm³ mol^-1) have been determined by spectrophotometry for the substitution in cobalt(III) corrinoids of co-ordinated H₂O by various amines B with different trans ligands X in aqueous solution (I = 0.2 mol dm^-3) at 25 °C: X = 5,6-dimethylbenzimidazole (dbzm), B = NH₂Me (ca. 6), NHMe₂ (3.4), NMe₃ (≤1), NH₂Et (5.3), NH₂Prⁱ and NH₂Bu^t (both ≤1); X = CN^-, B = NH₂Me (3.4), NHMe₂(2.0), NMe₃(ca. 0); X = vinyl, B = NH₃ (-0.06), NH₂Me (+0.04), NHMe₂ and NMe₃ (both ≤ - 1). These results show that steric effects increase in importance as the donor power of the trans ligand X decreases (from vinyl through CN^- to dbzm) and that significant steric effects are caused by substitution on the β (C) atom as well as on the α (N) atom.