Formation of chloro complexes of manganese(II), cobalt(II), nickel(II) and zinc(II) in dimethyl sulphoxide

Abstract

The formation of chloro complexes of MnII, CoII, NiII and ZnII in dimethyl sulphoxide (DMSO) has been studied by calorimetry and spectrophotometry at 25°C. The formation constants, enthalpies and entropies for [MCln](2–n)+(n= 1–4; M = Mn, Co, Ni, Zn) were determined and electronic spectra for the individual CoII and NiII complexes were extracted. It is revealed that the coordination structure of [CoCl]+, [NiCl]+ and [NiCl2] is octahedral and that of [CoCl2], [CoCl3]–, [CoCl4]2–, [NiCl3]– and [NiCl4]2– is tetrahedral. Thus, an octahedral to tetrahedral geometry change occurs at the second step for CoII and at the third step for NiII. The log K1 values for the formation of [MCl]+ vary in the order Mn > Co > Ni Zn, which is different from the Irving–Williams series. The overall log β4 values follow the sequence Mn Ni < Cu < Zn. Complexation entropies of these metal systems are appreciably smaller in DMSO than in N,N-dimethylformamide (DMF), which is ascribed to stronger solvent–solvent interactions in DMSO.