Thermal behaviour, phase transition and molecular motions in [Co(NH 3) 6](NO 3) 2

Abstract

At room temperature [Co(NH 3) 6](NO 3) 2 possesses a regular (cubic) crystal lattice structure (space group F m 3 ¯ 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 T C h = 229.7 K (during heating) and at T C c = 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 ν 4 ( N O 3 − ) E mode near the phase transition temperature T C c 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 ρ w(NH 3) F 1u mode suggests that the phase transition is not connected with a fundamental change in the speed of NH 3 reorientational motions. The NH 3 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(NH 3) 6](NO 3) 2 to [Co(NH 3) 2](NO 3) 2 takes place in two steps and four out of six NH 3 molecules per formula unit are liberated. The second stage is connected with the liberation of two residual NH 3 molecules and with the simultaneous decomposition of the resulting Co(NO 3) 2, leading to the formation of gaseous products of the decomposition (O 2, H 2O, N 2, nitrogen oxides) and solid CoO.