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Fm3m) and K 3 PtH 5 (P 4/mbm), are presented and discussed with respect to the influence of the different strengths of Pt-H and Pd-H bonds and of the relative volume of the reduced (per formula unit) unit cells. The 'antisymmetric' MD (M = Pt; Pd) stretching mode (species B 1g of free, square MX 4 units) decreases from 1436 to 1291 cm −1 on going from K 2 PtD 4 to Rb 2 PdD 4 . Likewise, the PtH stretch of Na 2 PtH 4 at 2026 cm −1 decreases to 2001 cm −1 for K 2 PtH 4 but increases to 2039 cm −1 for K 3 PtH 5 . The totally symmetric MH stretches (A 1g ), which are of lower intensity, are shifted by about 50 cm −1 to higher wavenumbers compared with the asymmetric stretches. The results of a group theoretical treatment of the respective site group and unit-cell group modes are given.

Crystal structure, DRIFT, infrared and Raman spectra, and the results of thermal analyses of the hitherto wrongly as Mg(H4I 0 6)2 • 4H20 and M g(I04) • 8H20 described dimesoperiodate MgH4I2O l0 • 6H20 and of the isostructural zinc compound are presented. The compounds crystallize in the monoclinic space group P2, (Z = 2) with a = 1071.0(2), b = 547.0(1), c = 1194.9(2) pm, and ß = 112.58(3)° and a = 1073.3(3), b = 545.3(2), c = 1188.3(5) pm, and ß = 112.52(3)°, respectively. The structure, which was refined from X-ray single crystal data of the magnesium compound (R 1 = 2.72%, 3824 independet reflections}, is built up from isolated distorted M(H20 ) 62+ octahedra and dimesoperiodate ions H4I2O 10 -connected by a network of hydrogen bonds formed by the H4I2O 10"~ ions and six crystallographically different hydrate H20 molecules. The strength of the hydrogen bonds ranges from unusually weak bonds corresponding to uncoupled (isotopically dilute samples) OD stretching modes of > 2600 cm-1 and ver^ strong ones (uOD: < 2200 cm-1). The IO stretching modes of the transconfigurated H4I2O l0~-ions are assigned to terminal 1-0 groups (816 cm-1 ), I-OH groups (746 and 762 cm' 1) and bridging 1-0 groups (618 and 647 cm-1). On heating, MgH4I2O |0 • 6H20 undergoes dehydration in the range of 373 -485 K (Differential Scanning Calorimetry) to two different polymorphs of magnesium metaperiodate (H4I2O |02--» ■ 2 I0 4-+ 2H20). Anhydrous M g(I04)2 is instable. Above 423 K (high-temperature Raman data), it decomposes to magnesium iodates.