Experimental and theoretical evaluation of magnetic coupling in organometallic radicals: the eloquent case of face-to-face CpCp interactions

The solid state magnetic properties of an extensive series of neutral radical (S = ½) complexes associating cyclopentadienyl (Cp) and dithiolene ligands and formulated as [CpNi(dithiolene)]˙, are successfully rationalized through a combination of structural analysis of the crystal structures and broken symmetry DFT calculations. The highly delocalized spin density of these complexes allows for strong antiferromagnetic interactions between radical species, which involve not only short intermolecular SS contacts but also SCp and CpCp contacts, demonstrating that the cyclopentadienyl moiety can effectively act as a non-innocent ligand in metal complexes where it bears a sizeable fraction of the spin density, for example, up to 20% in these [CpNi(dithiolene)]˙ neutral radical complexes.