The effect of pressure on the complex formation of cobalt (II) and nickel (II) bromides in acetone solution

Abstract

In acetone solution of CoBr_2 alone, the species of CoBr_2S_2 is predominant in the concentration range higher than ca. 10^-^4 mol/l at atmospheric pressure and room temperature, where S denotes an acetone molecule. With increasing pressure, the following two equilibria became detectable ; CoBr_2S_2 + 4S ⇌ CoS_6^2^+ + 2Br^- (I) CoBr_2S_2 + Br^- ⇌ CoBr_3S^- + S. (II) In acetone solution of CoBr_2 containing LiBr in large quantity, there exists the following equilibrium ; CoBr_3S^- + Br^- ⇌ CoBr_4^2^- + S. (III) and NiBr_2 has the correspondieg equilibrium under the same condition ; NiBr_3S^- + Br^- ⇌ NiBr_4^2^- + S. (IV) The pressure effect on these equilibria up to 5000 kg/cm^2 at 25℃ was studied by measuring the absorption spectra in the visible region and calculating the equilibrium constants at each pressure. The volume changes for the stepwise equilibria (I), (II) and (III) were obtained yo be -109±10, -5.2 and -0.8 cm^3/mol and for (IV) -4.3 cm^3/mol, respectively. The largely negative value for equilibrium (I) would be reasonable from the consideration of the electrostrictive effect.