δ-Bonding modulates the electronic structure of formally divalent nd¹ rare earth arene complexes

Landmark advances in rare earth (RE) chemistry have shown that divalent complexes can be isolated with non-Aufbau 4f ⁿ{5d/6s} ¹ electron configurations, facilitating remarkable bonding motifs and magnetic properties. We report a series of divalent bis-tethered arene complexes, [RE(NHAr ^{iPr ₆} ) ₂] (2RE; RE = Sc, Y, La, Sm, Eu, Tm, Yb; NHAr ^{iPr ₆} = {N(H)C ₆H ₃-2,6-(C ₆H ₂-2,4,6-iPr ₃) ₂}). Fluid solution EPR spectroscopy gives g _iso < 2.002 for 2Sc, 2Y, and 2La, consistent with formal nd ¹ configurations, calculations reveal metal-arene δ-bonding via mixing of nd _{(x ²−y ²)} 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 _{(x ²−y ²)} contribution to the SOMO. This contrasts 4f ⁿ{5d/6s} ¹ complexes where the valence d-based electron resides in a non-bonding orbital. Complexes 2Sm, 2Eu, 2Tm, and 2Yb contain highly-localised 4f ⁿ⁺¹ 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 ¹ and 4f ⁿ⁺¹ configurations are nuanced, may be controlled through ligand modification, and require a multi-pronged experimental and theoretical approach to fully rationalise.