Calorimetric and spectrophotometric studies of complexation of manganese(II), cobalt(II) and nickel(II) with bromide ions in N,N-dimethylformamide

Abstract

The complexation of manganese(II), cobalt(II) and nickel(II) with bromide ions has been calorimetrically and spectrophotometrically studied in N,N-dimethylformamide (DMF) containing 0.16 mol dm^–3(C₂H₅)₄NClO₄ as a constant ionic medium at 25 °C. The data obtained are well explained in terms of the formation of [MnBr]⁺ and [MnBr₃]^– for Mn^II, [CoBr]⁺, [CoBr₃]^– and [CoBr₄]^2– for Co^II and [NiBr]⁺ for Ni^II, and their formation constants, enthalpies and entropies have been determined. The formation of [MBr₂] is practically negligible in all the metal systems examined. Electronic spectra of individual complexes have also been extracted for Co^II and Ni^II. The coordination geometry surrounding manganese(II) and cobalt(II) ions changes upon complexation, and the change is strongly correlated to the reaction enthalpies and entropies. The coordination geometry thus postulated is octahedral for [MnBr(DMF)₅]⁺, [CoBr(DMF)₅]⁺ and [NiBr(DMF)₅]⁺, and tetrahedral for [MnBr₃(DMF)]^–, [CoBr₃(DMF)]^– and [CoBr₄]^2–.