Structure, molecular motion, and phase transition of a highly disordered crystal [Co(NH3)6](ClO4)3

Abstract

Four crystalline phases of the coordination compound [Co(NH₃)₆](ClO₄)₃ are identified by adiabatic calorimetry. An order–disorder phase transition (II→I) occurs at T_C1 = 334.2 K with an entropy change of 6.4 J K⁻¹ mol⁻¹. The X-ray single-crystal diffraction at 340 K demonstrates that phase I is cubic (Z = 4) and that two types of anions exist with different types of disorder. In phase II at 200 K, five anions (out of twelve) become ordered and three cations (out of four) are deformed to give a lower symmetry, still in a cubic system (Z = 32). This is attributed to orientational ordering of the anions triggered by NH⋯O hydrogen-bonding interactions. The ¹H-NMR study suggests that some NH₃ ligands are oriented because of hydrogen bonding, whereas some cations reorient isotropically like in phase I. The energy required to reorient an ordered anion in the crystal lattice of phase II is estimated from the excess heat capacity below T_C1 to be 40 kJ mol⁻¹, which corresponds to the energy needed to break the hydrogen bond, 8 kJ mol⁻¹. While the transitions at 111.7 K (III→II) and 97.6 K (IV→II), with an entropy change of 5.7 J K⁻¹ mol⁻¹, do not substantially affect the dynamics, they are attributed to further orientational ordering of the anions still disordered in phase II.