Interpretation of the temperature dependence of the EPR spectrum of Cu2+-doped (NH4)2[Cd(NH3)2(CrO4)2] and crystal structures of the high- and low-temperature forms of the host lattice

The crystal structure of (NH₄)₂[Cd(NH₃₎2(CrO₄)₂] is reported. Below about 300 K the compound changes from a monoclinic cell (space group C2/m, Z = 2, a = 12.8380(11) Å, b = 6.0308(6) Å, c = 7.5890(6) Å, β = 110.154(14)°) in which all four Cd-O bonds of the trans-Cd(NH₃)₂O₄ coordination sphere are crystallographically equivalent to a triclinic cell (space group P1, Z= 1, a = 6.0210(4) Å, b = 7.0363(4) Å, c = 7.5714(8) Å, α = 106.802(18); β = 93.032(12); γ = 114.079(11)°) in which the only symmetry element of the Cd complex is an inversion center. It is shown that the previously reported temperature dependence of the EPR spectrum of ~0.3% Cu²⁺ doped into this compound is consistent with the change in crystal structure. The spectra may be explained using a model of dynamic vibronic coupling in which the effects of Jahn-Teller coupling and a "strain" due to the inequivalence of the ligands are applied to the e_g vibrational and E_g electronic wave functions of the Cu²⁺ ion. The balance between the ligand field asymmetry and the natural tendency of Cu²⁺ to adopt a tetragonally elongated octahedral coordination geometry results in a complex with an orthorhombic coordination geometry having short bonds to the ammine groups and intermediate and long bonds to the chromate oxygen atoms. However, the long Cu-O bonds may occur to either pair of trans chromate oxygen atoms. In the high-temperature monoclinic unit cell, the EPR spectrum confirms that these two conformations are energetically equivalent, but in the low-temperature triclinic cell this is no longer the case, and the EPR spectrum is consistent with a temperature-dependent equilibrium between the two possible structural isomers. However, the model suggests that significant delocalization of the vibronic wave functions may occur, so that it is difficult to define precisely the bond lengths and electronic wave function parameters of the guest copper(II) complexes.