The highly resolved electronic spectrum of the square planar CuCl₄²⁻ ion

The low temperature magnetic circular dichroism(MCD) and electron paramagnetic resonance(EPR)spectra of Cu(II) dopedCs₂ZrCl₆ are reported. The Cu(II) ion is incorporated as the square planar copper tetrachloride ion, CuCl₄²⁻, which substitutes at the Zr(IV) site in the Cs₂ZrCl₆ lattice, with a complete absence of axial coordination. Both the EPR and MCD show highly resolved spectra from which it is possible to determine the superhyperfine coupling constants and excited state geometries respectively. The Franck–Condon intensity patterns suggest that there is a substantial relaxation of the host lattice about the impurity ion. For the lowest energy ²B1g(x²-y²)→²B2g(xy) transition, both the magnetic dipole allowed electronic origin as well as vibronic false origins are observed. The high resolution of the spectra allowed the accurate determination of the odd parity vibrations that are active in the spectra. The opposite sign of the MCD of the two components of the ²Eg(xz,yz)excited state allows this splitting to be determined for the first time. Accurate and unambiguous spectral parameters for the CuCl₄²⁻ ion are important as it has become a benchmark transition metal complex for theoretical electronic structure calculations