The crystal structures at 10 K of the trigonal salts KMIII(NH3)(6)(ClO4)(2)Cl-2, M-III = Os and Cr, and the possibility of phase transitions

The crystal structures of KOs(NH3)(6)(ClO4)(2)Cl-2 and KCr(NH3)(6)(ClO4)(2) Cl-2 were determined at 10 K by X-ray diffraction, and for the osmium salt also at 293 K. At 293 K the osmium salt is trigonal, space group <R(3)over bar>m, with the same simple structure as others of this class of double sal t. At 10 K, in agreement with previous radius ratio predictions, both cryst als are best described as remaining <R(3)over bar>m. All previously studied members, with larger alkali metal cations, are twinned <R(3)over bar> at l ow temperatures, with small, symmetry-breaking rotations of the hexaamminem etal(III) and perchlorate ions about the threefold axis. Differential scann ing calorimetry on CsRu(NH3)(6)(ClO4)(2)Cl-2 suggests that the <R(3)over ba r>m to <R(3)over bar> change is very extended in temperature with only a sm all discontinuity at the transition temperature.