Spin density accuracy and distribution in azido Cu(II) complexes : a source function analysis

Magnetic properties of open-shell systems depend on their unpaired electron density distribution. Accurate spin density (SD) is difficult to retrieve, both from polarized neutron diffraction (PND) data and from quantum approaches, and its interpretation is not trivial. The Source Function is a useful tool to interpret SD distributions and their accuracy. It is here applied to analyze and compare the theoretical SD in a weakly ferromagnetically coupled end-end azido dicopper complex with that in a strongly-coupled end-on complex. The Source Function enables to highlight the origin of the SD differences between the two dicopper complexes and among adopted computational approaches (CASSCF, DFT, UHF). Further insight is provided by partial Source Function SD reconstructions using given subsets of atoms. DFT methods exaggerate electron sharing between copper and the ligands, causing spin delocalization toward them and overestimating metal-ligand spin polarization, while underestimating CASSCF spin information transmission between atoms. CAS(10,10) SD is closer to the PND SD than other adopted methods.