Electron spin resonance spectra and covalent bonding in the square-planar CuCl42− and CuBr42− ions

Electron paramagnetic resonance spectra of copper‐doped potassium tetrachloropalladate and potassium tetrabromopalladate have been observed. Both hyperfine and superhyperfine structures are resolved at room temperature. The ${\mathrm{CuCl}}_4^{\text{2−}}$ and ${\mathrm{CuBr}}_4^{\text{2−}}$ units are in a square‐planar configuration. The data can be fitted with an axial spin Hamiltonian. The parameters are as follows; g_∥ = 2.232, $g_{\perp }\text{=2.049}$ , $A_{\parallel }^{\mathrm{Cu}}{\text{=163.6 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\perp }^{\mathrm{Cu}}{\text{=34.5 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\parallel }^{\mathrm{Cl}}{\text{=23.3 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , and $A_{\perp }^{\mathrm{Cl}}{\text{=5.3 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ for ${\mathrm{CuCl}}_4^{\text{2−}}$ and g_∥ = 2.143, $g_{\perp }{\text{=2.0438, A}}_{\parallel }^{\mathrm{Cu}}{\text{=189.5 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\perp }^{\mathrm{Cu}}{\text{=45.8 ×}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\parallel }^{\mathrm{Br}}{\text{=123 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , and $A_{\perp }^{\mathrm{Br}}{\text{=27.9 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ for ${\mathrm{CuBr}}_4^{\text{2−}}$ . The MO parameters are evaluated exactly. There are strong covalent in‐plane σ and π bonds and out‐of‐plane π bonds. The covalency is larger for Br⁻ than Cl⁻ as a ligand. Data were also obtained on single crystal copper‐doped tetraphenylarsonium trichloropalladate. The pure salt contains ${\mathrm{Pd}}_2{\mathrm{Cl}}_6^{\text{2−}}$ dimers, and the EPR spectra are consistent with ${\mathrm{PdCuCl}}_6^{\text{2−}}$ dimeric units in the doped species. The spin Hamiltonian parameters are essentially the same as in ${\mathrm{CuCl}}_4^{\text{2−}}$ , with g_∥ = 2.245, $g_{\perp }\text{=2.045}$ , $A_{\parallel }^{\mathrm{Cu}}{\text{=155 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\perp }^{\mathrm{Cu}}{\text{=57 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , $A_{\parallel }^{\mathrm{Cl}}{\text{=23.8 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ , and $A_{\perp }^{\mathrm{Cl}}{\text{=5.2 × 10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ . Dimerization does not change significantly the character of the Cu–Cl bonds.