δ-Bonding modulates the electronic structure of formally divalent nd rare earth arene complexes.

Landmark advances in rare earth (RE) chemistry have shown that divalent complexes can be isolated with non-Aufbau 4f {5d/6s} electron configurations, facilitating remarkable bonding motifs and magnetic properties. We report a series of divalent bis-tethered arene complexes, [RE(NHAr )] (2RE; RE = Sc, Y, La, Sm, Eu, Tm, Yb; NHAr = {N(H)CH-2,6-(CH-2,4,6-Pr)}). Fluid solution EPR spectroscopy gives < 2.002 for 2Sc, 2Y, and 2La, consistent with formal nd configurations, calculations reveal metal-arene δ-bonding mixing of nd valence electrons into arene π* orbitals. Experimental and calculated EPR and UV-Vis-NIR spectroscopic properties for 2Y show that minor structural changes markedly alter the metal d contribution to the SOMO. This contrasts 4f {5d/6s} complexes where the valence d-based electron resides in a non-bonding orbital. Complexes 2Sm, 2Eu, 2Tm, and 2Yb contain highly-localised 4f ions with no appreciable metal-arene bonding by density functional calculations. These results show that the physicochemical properties of divalent rare earth arene complexes with both formal nd and 4f configurations are nuanced, may be controlled through ligand modification, and require a multi-pronged experimental and theoretical approach to fully rationalise.