Pressure effects on the complexes of cobalt (II) chloride and cobalt (II) bromide in acetone solution

Abstract

The acetone solutions of cobalt(II) chloride and cobalt(II) bromide are blue under atmospheric pressure at room temperature and in both of them the main species is tetrahedrally coordinated CoX_2(Ac)_2, where Ac denotes an acetone molecule and X is Cl or Br. The visible absorption spectra of cobalt(II) chloride and cobalt(II) bromide in acetone solution measured under pressures up to 8, 000kg/cm^2 at room temperature showed that the following two kinds of equilibria coexist : CoX_2(Ac)_2 + 4Ac ⇌ Co(Ac)_6^2^+ + 2X^-, (I) CoX_2(Ac)_2 + X^- + ⇌ CoX_3(Ac)^- + Ac. (II) The value of ΔV_1, the volume change of equilibrium (I), is large with the negative sign and changes greatly with increasing pressure. |ΔV_2|, the absolute value of the volume change of equilibrium (II), is small and scarcely depends on pressure. These experimental results indicate the nature of equilibrium (I) and eqilibrium (II): in equilibrium (I) the ionic species are formed and the coordination number increases by the shift to the right side, and in equilibrium (II) there is no change in the number of the charged species and in the coordination number on both sides. In addition, ΔV_1 was estimated from the change of the intrinsic volume, the free volume and the effect of electrostriction.