Phase Transitions and Structures at 10, 92 and 293 K in Eight Trigonal Double Salts M^I[M^III(NH₃)₆](ClO₄)₂X₂

The crystal structures of eight hexaammine-M^III double salts have been determined by X-ray diffraction at 293 K, where M^I, M^III and X are NH₄, Co, Cl (I), ammonium hexaamminecobalt(III) diperchlorate dichloride; Cs, Co, Cl (II), caesium hexaamminecobalt(III) diperchlorate dichloride; NH₄, Ru, Cl (III), ammonium hexaammineruthenium(III) diperchlorate dichloride; K, Ru, Cl (IV), potassium hexaammineruthenium(III) diperchlorate dichloride; Rb, Ru, Cl (V), rubidium hexaammineruthenium(III) diperchlorate dichloride; Cs, Ru, Cl (VI), caesium hexaammineruthenium(III) diperchlorate dichloride; Cs, Cr, Cl (VII) caesium hexaamminechromium(III) diperchlorate dichloride; Cs, Cr, Br (VIII), caesium hexaamminechromium(III) diperchlorate dibromide. The structures of (I), (IV), (V) and (VI) have also been determined at 92 K and those of (V), (VII) and (VIII) at 10 K. At room temperature all are isomorphous, trigonal R3m, Z = 3, with only seven positional parameters for the seven unique non-H atoms. The ammine H atoms are disordered only in the Cs⁺ and NH salts, and the NH H atoms in the ammonium salts. At lower temperatures a reversible phase change is observed in all except the K⁺ member, by the lowering of symmetry to a twinned, but atomically ordered, R3 phase. In this R3 phase the hexaammine-M^III and perchlorate ions have rotated about the threefold axis removing the former mirror plane in the crystal. Displacement parameters, unit cells and observed structures all change smoothly with changes in M^I ionic radii and M^III-N distances.

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