Oscillations, Waves, and Interactions - GWDG

352 R. Pottel, J. Haller and U. Kaatze
Figure 17. Numbers Z + of apparently irrotationally bound water molecules per ion for
some mono-, di-, and trivalent cations [21] and graphical representation of the extent of
dielectric saturation: hatched areas show the saturated water shell around the ions the
crystallographic diameters of which are indicated by discs.
the solution. Quite remarkably the Z + values of the transition metal ions Zn 2+ , Cd 2+
and In 3+ are considerably smaller than those of the similarly sized Ca 2+ , Sr 2+ , and
Y 3+ ions, respectively. These small Z + values have been taken an indication of cationanion
complex formation in the salt solutions containing transition metal ions. In
vacuum the electronic configuration of such ions with complete d shell, like that of
main group metal ions, involves a spherical charge distribution. In solution, however,
interactions with anions are likely promoted by the directed d 10 electron orbitals, thus
increasing the tendency towards complex formation even with monovalent anions.
The effect of dielectric saturation is smaller around the ion complex structures with
reduced electric field strength than around separated ions with their comparatively
strong Coulombic field. Therefore, the decrement in ɛ(0) is smaller in solutions
with d 10 cations than with equally sized d 0 cations. Complex formation in aqueous
solutions of salts from transition metal cations and monovalent anions has been
verified by solute contributions to the complex dielectric spectrum [21] and also by
ultrasonic excess absorption spectra. An exceptional example is discussed in the next
section.