Electronic properties of D 4h d 9 complexes through a crystal-field model: an appraisal

Abstract

The crystal-field model recently used by Huang et al. [J. Phys. Chem. Solids 64 (2003) 523] for explaining some optical transitions and the g and hyperfine tensors of the CuCl 4 2− unit placed in K 2PdCl 4 is critically reviewed. It is firstly pointed out that using accurate 〈 r 4〉 0=2.7 a.u. and 〈 r 2〉 0=1.0 a.u. values for free Cu 2+ the d–d transitions of CuCl 4 2− calculated in a rigorous crystal-field framework are much smaller than experimental ones. The agreement with experimental values when an approximated 3d wavefunction is used (leading to 〈 r 4〉 0=44.8 a.u. should then be taken as meaningless. The neglect of charge transfer (CT) transitions for explaining the experimental EPR parameters is shown to be against the onset of these excitations in CuCl 4 2− observed at about 25,000 cm −1 and the 40% delocalization of the unpaired electron in the b 1 g (≈ x 2− y 2) level. Finally it is pointed out that the experimental g tensor for similar units like AgBr 6 4− or CuBr 4 2− involving a higher covalency cannot be explained by a model where the contribution of CT transitions and ligand spin–orbit coupling is ignored.