Electronic properties of D4h d9 complexes through a crystal-field model: an appraisal

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 CuCl42− unit placed in K2PdCl4 is critically reviewed. It is firstly pointed out that using accurate 〈r4〉0=2.7 a.u. and 〈r2〉0=1.0 a.u. values for free Cu2+ the d–d transitions of CuCl42− 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 〈r4〉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 CuCl42− observed at about 25,000 cm−1 and the 40% delocalization of the unpaired electron in the b1g (≈x2−y2) level. Finally it is pointed out that the experimental g tensor for similar units like AgBr64− or CuBr42− involving a higher covalency cannot be explained by a model where the contribution of CT transitions and ligand spin–orbit coupling is ignored.