Stabilities of binary and ternary complexes of divalent metal ions with tiron and 2,2′-bipyridyl

Abstract

The formation constants have been measured for the mixed complexes MBT and MBT(OH), where M = Mn(II), Co(II), Ni(II), Zn(II), Cd(II), B = 2,2′-bipyridyl and T = tiron, by computer-aided pH-potentiometry at 25°C and 0.1 M potassium nitrate. The parent complexes with tiron have also been studied, giving species such as MT, Mt 2, MT 3, MHT and MT 2(OH). The stability constants of the parent and mixed complexes follow the Irving-Williams sequence. For all mixed complexes an increase in stability compared to the statistical case was found and characterized by the re-proportionation constant X. The high stability of mixed complexes is attributed to π back-bonds between the 3d-metal ions and the π systems of the ligands.