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

The formation of chloro complexes of Mn^II, Co^II, Ni^II and Zn^II in dimethyl sulphoxide (DMSO) has been studied by calorimetry and spectrophotometry at 25 °C. The formation constants, enthalpies and entropies for [MCl_n]^(2-n)+ (n = 1-4; M = Mn, Co, Ni, Zn) were determined and electronic spectra for the individual Co^II and Ni^II 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 Co^II and at the third step for Ni^II. The log K₁ values for the formation of [MCl]⁺ vary in the order Mn > Co > Ni < Cu > Zn, which is different from the Irving-Williams series. The overall log β₄ values follow the sequence Mn < Co > 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.