The chemistry of vitamin B₁₂. Part 21. Ethynylaquocobinamide: Novel reaction of diaquocobinamide with acetylene catalysed by copper ions

Copper ions catalyse the reaction of diaquocobinamide with acetylene in aqueous solution at room temperature to give ethynylaquocobinamide as a mixture of the two isomers which differ in the relative orientation of the axial ligands. This leads to the first reported value for an equilibrium constant (axial ligands only given) involving the substitution of co-ordinated H₂O by HC₂^- of K = [HC₂-Co-OH₂]/([H₂O-Co-OH₂] [HC₂^-]) ≥ 10²³ dm³ mol^-1. Equilibrium constants have been determined for the substitution of co-ordinated H₂O in ethynylaquocobinamide by OH^- (pK = 13.0 ± 0.1), CN^- (log₁₀K ≥ 6.8), and imidazole (log₁₀K = 3.4), which confirm the position of HC₂^- between cyanide and vinyl in the trans-effect series for Co^III corrinoids.