Crystal Structures and Pressure-Induced Redox Reaction of Cs₂PdI₄·I₂ to Cs₂PdI₆

Two iodopalladates of the same empirical formula with palladium in different oxidation states were synthesized from aqueous HI solution. Their crystal structures were characterized by single-crystal X-ray analysis, and the effect of hydrostatic pressure on the structural properties has been investigated. Dicesium hexaiodopalladate(IV), Cs₂PdI₆, crystallizes in a cubic system, space group Fm3̄m, with a = 11.332(1) Å and Z = 4, and is isotypic to K₂PtCl₆. The second compound, dicesium tetraiodopalladate(II) diiodine, Cs₂PdI₄·I₂, shows tetragonal symmetry with space group I4/mmm, a = 8.987(1) Å, c = 9.240(1) Å, and Z = 2. The crystal structure can be described in resemblance to the Cs₂Au^IAu^IIICl₆ type. Structural relationships and chemical and structural transformation between both compounds will be discussed. DTA measurements at ambient pressure showed liberation of I₂ and decomposition of the compounds. Cs₂PdI₄·I₂ represents an excellent example for studying a solid-state electron-transfer reaction. The redox reaction to Cs₂PdI₆ can be demonstrated by performing pressure-dependent X-ray studies.