Molar volumes of zinc(II) thiocyanato complexes and volume changes of complexation in N,N-dimethylformamide

Abstract

Molar volumes of zinc(II) thiocyanato complexes have been evaluated by densitometric titration in N,N-dimethylformamide at 298.15 K. All the measurements were carried out in solution containing an inert supporting electrolyte 0.2 mol dm⁻³ (C₂H₅)₄NClO₄ and the molar volumes of all the ionic species were assumed to be kept constant throughout titration. The volume changes, ΔV, thus determined for the reactions are as follows:

Zn²⁺ + SCN⁻ = [Zn(NCS)]⁺; ΔV°_0–1 = 15 cm³ mol⁻¹ (1)

[Zn(NCS)]⁺ + 2SCN⁻ = [Zn(NCS)₃]⁻; ΔV°_1–3 = 77cm³ mol⁻¹ (2)

[Zn(NCS)₃]⁻ + SCN⁻ = [Zn(NCS)₄]²⁻; ΔV°_3–4 = −18 cm³ mol⁻¹ (3)

These show that the volume changes parallel the corresponding reaction entropies. The ΔV_1–3^° of reaction (2) shows a relatively large value, as expected from the fact that the coordination geometry around the zinc(II) ion changes from octahedral six-coordination to tetrahedral four-coordination.