Electron paramagnetic resonance of penta-ammino copper fluoroborate: coexistence of isotropic and anisotropic spectra in tetragonal phase

Abstract

The results of polycrystalline electron paramagnetic resonance (EPR) on tetra- and penta-ammino copper(II) fluoroborate are reported. The EPR spectra of penta-ammino Cu(II) complexes in perchlorate and fluoroborate [Cu(NH3)5]X2· xNH3 depend on the presence of ‘lattice ammonia’, x, the number of ammonia molecules per Cu(II) ion above n = 5 that are not coordinated to Cu(II). In the high temperature cubic phase the spectrum is always an isotropic line. Furthermore, there is a substantial dependence of the averaged g-factor on the amount of ‘lattice ammonia’. In the tetragonal phase of [Cu(NH3)5](BF4)2·0.1NH3, below Tc, there are two components to the EPR spectrum: the anisotropic one due to the square pyramid coordination and the isotropic one due to the centres where the averaging mechanism is still efficient. The contribution of the averaged signal to the total EPR spectrum in fluoroborate is significantly different than in isomorphous perchlorate salt. The higher content of isotropic signal in the total EPR spectrum suggests that fluoroborate exhibits a stronger tendency to retain the cubic structure in the low temperature phase than perchlorate.