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Single-crystal Raman spectra in the range 4000-35 cm −1 and the results of group theoretical treatment including Cartesian symmetry coordinates of nickel iodate dihydrate, Ni.IO 3 / 2 · 2H 2 O, (space group Pbca, Z = 4) are presented and discussed with respect to the strength of the HO -H· · ·OIO 3 hydrogen bonds and the extent of intramolecular (intraionic) coupling of the stretching vibrations of both the hydrate H 2 O molecules and the iodate IO 3 − ions. The energies and relative intensities of the IO stretching modes in the various species of the unit-cell group D 2h reveal that the iodate ions of the title compound behave as if they possess pseudo C 2v site symmetry with a at least partly uncoupled IO stretch due to the significantly shortest (175.5 pm) IO arm and a symmetric (n s ) and an antisymmetric stretching vibration (n as ) due to the remaining IO 2 group (r I -O = 184.2 and 186.0 pm). The respective site-group modes decrease in the order n 3a $ n IO.1/ (mean value of the respective unit-cell group modes: 804 cm −1 / > n 1 $ n s (772 cm −1 / > n 3b $ n as (744 cm −1 ) (n 1 and n 3 as for distorted C 3v IO 3 − ions). For the IO 3 − bending modes (430-300 cm −1 ) and the lattice modes below 300 cm −1 the true vibrational modes are not known. The normally used assignment to a symmetric (n 2 ) and an asymmetric bending vibration (n 4 ) as for C 3v IO 3 − ions is at least questionable.The hydrogen-bond distances calculated from the wavenumbers of the OD stretching modes of matrixisolated HDO molecules do not fully agree with the data of a neutron diffraction study performed in 1972.

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