The chemistry of vitamin B12. Part 30. Co-ordination of amines, pyridines and diazines by aquacyanocobinamide; nitrogen-containing bases as probes for basicity and other effects in metal–ligand bonding

Abstract

Equilibrium constants K have been determined by UV/VIS spectrophotometry for the substitution of co-ordinated H₂O in the cobalt(III) corrinoid aquacyanocobinamide by NH₃, four primary alkylamines (pK values 5.3–8.5), NH₂NH₂, NH₂OH, pyridine, four 4-substituted pyridines (pK values 1.9–9.8) and three diazines in aqueous solution with I ca. 0.1 mol dm^–3 at 25 °C. The results demonstrate that both the amines and the six-membered heterocycles obey the linear free energy relationship log K=a·pK+b, where a= 0.58 and b=–2.6 for the amines and a= 0.47 and b=+0.2 for the azines, and that NH₂NH₂, NH₂OH and the 1,2-diazine pyridazine all show enhanced values of log K which can be ascribed to operation of the α effect. Present and published data have been summarised to show that bases belonging to the three families (amines, six-membered heterocycles end five-membered heterocycles) can routinely be used to probe the role of at least four factors (basicity, the ‘group-specific’ factors which distinguish the three families, the α effect and steric effects) in metal–ligand bonding.