Ionisation ratios for the protonation of a series of isothiocyanato-complexes of cobalt(III) and rhodium(III) and the protonation of azidocomplexes in aqueous solutions of different acids

Abstract

The protonation of a series of isothiocyanatocobalt(III) complexes has been studied and the apparent pK_BH⁺ values are reported. The series was chosen so that the effects of other groups within the co-ordination sphere, e.g., the degree of chelation and chelate ring size could be investigated. The behaviour of the complex ions [M(NH₃)₅NCS]²⁺(where M = Rh and Co) has been compared. The pK_BH⁺ value of the complex is governed mainly by the particular ligand which is protonated, i.e., isothiocyanato- or azido-group. Other factors, e.g., chelation, and central metal ion do influence pK_BH⁺ values, but only slightly. The protonation of [Rh(NH₃)₅N₃]²⁺ ion, in different acids has been studied. The pK_BH⁺ value for this complex varies from acid to acid (as had been expected from the kinetically evaluated pK_BH⁺ values). This may be taken as evidence for the influence and importance of ion-pairing in these solutions.