The nickel(II)–bromide system in dimethyl sulphoxide: a detailed study of the influences of temperature and mole ratio

Abstract

The bromo-nickel(II) species occurring in dimethyl sulphoxide (dmso) solutions have been identified from their ligand field spectra and comprise the octahedral complexes [NiBr(dmso)₅]⁺ and [NiBr₂(dmso)₄], and the tetrahedral complexes [NiBr₃(dmso)]^– and [NiBr₄]^2–. The absorption spectra were recorded of solutions at various Br: Ni mole ratios, from 2 to 154, and from room temperature to 170 °C. Within separate temperature and Br: Ni mole ratio ranges five equilibria (1)–-(5) were found to predominate. For equilibrium (3) the enthalpy, ΔH, varied with Br: Ni mole, [Ni(dmso)₆]²⁺+ Br^–⇌[NiBr(dmso)₅]⁺+ dmso (1), [NiBr(dmso)₅]⁺+ Br^–⇌[NiBr₂(dmso)₄]+ dmso (2), [NiBr₂(dmso)₄]+ Br^–⇌[NiBr₃(dmso)]^–+ 3 dmso (3), [NiBr₃(dmso)]^–+ Br^–⇌[NiBr₄]^2–+ dmso (4), [NiBr₄]^2–+ dmso ⇌[NiBr₃(dmso)]^–+ Br^–(5) ratio, from 113.1 ± 1 kJ mol^–l at 8.4 to 47.2 ± 1 kJ mol^–l at 154.4, and a plot of ΔH vs. log₁₀(Br: Ni mole ratio) was linear. For equilibria (4) and (5)ΔH values were 43.7 ± 1 and –2.7 ± 1 kJ mol^–1, respectively. It is proposed that equilibrium (4) reverses to (5) at high temperatures, when the dielectric constant of the medium reduces sufficiently to destabilise the doubly charged [NiBr₄]^2– species. Such a reversal has not previously been reported. The free energy (ΔG) and entropy (ΔS) for equilibria (3)–(5) were obtained. The system is compared with the corresponding chloro system reported earlier. Several parallel general trends were noted, but there are many differences, brought out by this more detailed study.