PMR studies of blue form of anhydrous copper(II) formate

Abstract

PMR spectra were recorded on the single crystals of Cu(HCOO) 2·4H 2O and the blue modification of Cu(HCOO) 2. The spectra exhibit paramagnetic shifts due to the hyperfine coupling and the dipole field of Cu(II) ions. The angular dependence of the PMR shifts provides the following conclusions: 1) the crystal of blue Cu(HCOO) 2 has a layer structure consisting of anti-anti CuOCOCu bondings resembling that of the tetrahydrate, 2) the carbon atoms of the anhydride as well as of the tetrahydrate carry a negative spin density amounting to −0.025–−0.030 on their p orbitals, 3) in both of the formates, the spin delocalization takes place through the p π systems of the CuOCOCu bondings, and consequently 4) the superexchange interaction operates between Cu atoms through the π pathway which induces antiferromagnetic interaction in conformity with their magnetic properties.