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 CuCl42− and CuBr42− 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⊥=2.049, A∥Cu=163.6 × 10−4cm−1, A⊥Cu=34.5 × 10−4cm−1, A∥Cl=23.3 × 10−4cm−1, and A⊥Cl=5.3 × 10−4cm−1 for CuCl42− and g∥ = 2.143, g⊥=2.0438, A∥Cu=189.5 × 10−4cm−1, A⊥Cu=45.8 ×−4cm−1, A∥Br=123 × 10−4cm−1, and A⊥Br=27.9 × 10−4cm−1 for CuBr42−. 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 Pd2Cl62− dimers, and the EPR spectra are consistent with PdCuCl62− dimeric units in the doped species. The spin Hamiltonian parameters are essentially the same as in CuCl42−, with g∥ = 2.245, g⊥=2.045, A∥Cu=155 × 10−4cm−1, A⊥Cu=57 × 10−4cm−1, A∥Cl=23.8 × 10−4cm−1, and A⊥Cl=5.2 × 10−4cm−1. Dimerization does not change significantly the character of the Cu–Cl bonds.