On the sequence of three related phases of [Ni(H2O)2(15-crown-5)](HSO4)2 in the temperature range 110-295 K.

Attempts to prepare the compound [Ni(H(2)O)(2)(15-crown-5)](X)(2) were eventually successful with X = NO(3)(-) provided that a synthetic route aimed at restricting water was followed. Application of this method was extended to make the analogous compound with X = HSO(4)(-), for which three symmetry-related phases were isolated between 295 and 110 K: a room-temperature phase with Z' = (1/2) [phase (I)], an intermediate-temperature phase with Z' = 1 [phase (II)] and a low-temperature phase with Z' = 2 [phase (III)]. The phases are related by two reversible solid-solid phase transitions, and both transitions take place without a significant loss of crystallinity. In the phase sequence (I) <--> (II) <--> (III) (Z': (1/2) <--> 1 <--> 2), the crystal packing remains remarkably similar but the degree of order in the crystal changes significantly; the structure is very disordered at room and intermediate temperatures but is ordered at 110 K. The compound [Ni(H(2)O)(2)(15-crown-5)](HSO(4))(2) has a complicated hydrogen-bonding network, which contains O-H...O bonds between the counterions. Structural changes are largest along some face-diagonal directions in the sequence (I) <--> (II) <--> (III).