δ-Bonding modulates the electronic structure of formally divalent nd 1 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 6 H 3 -2,6-(C 6 H 2 -2,4,6- i Pr 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.