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)(4)NClO4 and the molar volumes of all the ionic species were assumed to be kept constant throughout titration. The volume changes, Delta V, thus determined for the reactions are as follows:
Zn2+ + SCN- = [Zn(NCS)](+); Delta V(0-1)degrees, = 15 cm(3) mol(-1) (1)
[Zn(NCS)](+) + 2SCN(-) = [Zn(NCS)(3)](-); Delta(1-3)degrees = 77 cm(3) mol(-1) (2)
[Zn(NCS)(3)](-) + SCN- = [Zn(NCS)(4)](2-); Delta V(3-4)degrees = -18 cm(3) mol(-1) (3)
These show that the volume changes parallel the corresponding reaction entropies. The Delta V(1-3)degrees 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.