Thermal behaviour, phase transition and molecular motions in [Co(NH3)6](NO3)2

At room temperature [Co(NH3)6](NO3)2 possesses a regular (cubic) crystal lattice structure (space group Fm3¯m) with the lattice parameter a = 11.0441 Å and with four molecules in the unit cell. One phase transition between 93 and 297 K, namely: at TCh=229.7 K (during heating) and at TCc=225.2 K (during cooling) was detected by differential scanning calorimetry. The presence of 4.5 K hysteresis suggests that the detected phase transition is of the first-order type. The splitting of the band connected with the ν4(NO3−)Eν4(NO3−)E mode near the phase transition temperature TCc suggests lowering of the crystal lattice symmetry. The lack of a sudden change in temperature dependence of the FWHM of the band connected with the ρw(NH3)F1u mode suggests that the phase transition is not connected with a fundamental change in the speed of NH3 reorientational motions. The NH3 ligands reorientate fast (correlation times equal to ca. several picoseconds) in both phases with an activation energy equal to ca. 11 kJ mol−1. Thermal decomposition of the compound proceeds in two main stages. In the first stage, deammination of [Co(NH3)6](NO3)2 to [Co(NH3)2](NO3)2 takes place in two steps and four out of six NH3 molecules per formula unit are liberated. The second stage is connected with the liberation of two residual NH3 molecules and with the simultaneous decomposition of the resulting Co(NO3)2, leading to the formation of gaseous products of the decomposition (O2, H2O, N2, nitrogen oxides) and solid CoO.