The low-temperature crystal structure (203 and 123 K) and electronic properties of diammonium hexa-aquacopper(II) disulphate: a fluxional CuO6 chromophore

Abstract

The low-temperature crystal structure of [NH₄]₂[Cu(OH₂)₆][SO₄]₂(1) has been determined by X-ray diffraction methods at 203 and 123 K, in the monoclinic space group, P2₁/a. The structure changes very little with temperature, except with respect to the geometry of the CuO₆ chromphore which becomes more elongated rhombic octahedral with decreasing temperature (298, 203, 123 K): Cu–O(7) 2.219(5), 2.250(2), 2.278(2); Cu–O(8) 2.095(5), 2.041 (2), 2.012(1); Cu–O(9) 1.961 (5). 1.967(2), 1.970(1)Å. These changes are consistent with a fluxional behaviour of two 90° misaligned elongated rhombic octahedral CuO₆ chromophores separated by less than thermal energy. From the relative thermal populations of the lowest two wells of the potential-energy surface at the three temperatures a Boltzmann distribution, least-squares fit, enables the separation of the two wells, ΔE, to be determined as 160 ± 20 cm^–1. From the temperature variations of the single-crystal g values (100–298 K) of (1) the ΔE value is estimated to be 168 ± 40 cm^–1, which is consistent with the crystallographic estimation of this parameter.