The paramagnetic centers of Cu2+ were studied at 300 and 77K, using X-band and Q-band electron paramagnetic resonance methods. The spin Hamiltonian parameters, contact interaction and covalent bonding parameters of the paramagnetic centers were determined. The superhyperfine interaction with the Cu paramagnetic centers in NH4Br was much stronger than that in NH4Cl. The results for the contact interaction, together with a new attribution for the electron paramagnetic resonance spectra, were in agreement with those found for other Cu complexes. A comparison of the results for the covalent bonding parameters, with those for other compounds, revealed a strong covalency of NH4Br:Cu2+; thus suggesting that orbital admixtures in this complex were large and apparently much greater than estimates based upon overlap effects alone, with no covalent bonding. The Cu2+ wave function of NH4Br, written as an admixture of |3z2−r2> and |x2−y2> orbitals, was unsuitable for describing the system. An effective model had to take account of admixtures with the ligand orbitals. The d–s mixing was high in the case of the CuBr4(NH3)22− center; thus indicating a strong bonding between the Cu2+ ion and the two NH3 molecules. In the case of CuBr4(NH3)(H2O)2− and CuBr4(H2O)22− centers, the binding between the Cu2+ ion and H2O molecules was much weaker.

Covalent Bonding in NH4Br:Cu2+. E.Di Mauro, S.M.Domiciano: Journal of Physics and Chemistry of Solids, 1999, 60[11], 1849-54