While the effect of rubidium and caesium ions in solid salts of IrCl6--is a shift to lower wave-numbers, comparable to that occurring in organic solvents relative to water, the dark blue silver (I) and thallium (I) salts have a new, broad band in the red. Mixed crystals of Cs2IrCl6and Tl2IrCl6are prepared, and the absorption spectra are discussed in terms of a tendency to ‘ferroelectric rattling’ and strong covalent bonding of Ag and Tl. Analogous cases are pointed out in solid salts of other 5d-hexahalides.