The chemistry of vitamin B12. Part 29. Co-ordination of imidazoles and 1,2,4-triazole by aquacyanocobinamide

Abstract

Equilibrium constants K for the substitution of co-ordinated H₂O in the cobalt(III) corrinoid aquacyanocobinamide by imidazole, 1,2,4-triazole and other azoles have been determined in aqueous solution with I ca. 0.1 mol dm^–3 at 25 °C by UV/VIS spectrophotometry. The results demonstrate that the azoles obey the linear free energy relationship log K=a·pK+b, where a= 0.42 and b= 1.3, over 12 pK units from 1,2,4-triazole (pK_b 2.3) to imidazolate (pK_a 14.3) and that determining the values of log K for just the four ligands imidazole, 1,2,4-triazole and their anionic forms (the latter via pK_c for deprotonation of the co-ordinated ligand) provides a general method for establishing the linear relationship and the values of a and b in labile complexes. It is shown that for kinetically inert complexes only the values of pK_c for deprotonation of co-ordinated imidazole and 1,2,4-triazole need to be determined in order to establish the linear relationship and to derive a value of a(but not of b), as in the case of [Ru(NH₃)₅(azole)]³⁺ complexes where published values of pK_c can be used to derive a value of a= 0.75.