s‑Block Metal-Lanthanide Bonding: Direct Comparison of Mg–Yb and Mg–Ca Complexes

Despite considerable progress in understanding the fundamental aspects of metal–metal bonding, lanthanide-metal bonding is limited to interactions between lanthanide (Ln) metals and electron-rich p- or d-block metals. Exemplified by a Mg–Yb complex, we present synthesis and extensive computational analyses of the first s-block metal-Ln bond. Key to its formation is the unique Mg0 complex [(BDI*)MgNa]2 (1) which can be viewed as a dimer consisting of two magnesyl anions, (BDI*)Mg–, bridged by two Na+ cations (BDI* = HC[C(tBu)N-DIPeP]2, DIPeP = 2,6-CHEt2-phenyl). Reaction of dimeric 1 with (YbN’’2)2 (N’’ = N(SiMe3)2) quantitatively gave the mixed complex [(BDI*)MgNa/YbN’’2] (2-Yb) in which (BDI*)Mg– anions bridge between Yb2+ and Na+ cations (Mg–Yb: 3.466(1), Mg–Na: 3.415(1) Å). Atoms-In-Molecules shows Mg–Yb and Mg–Na bond paths corresponding to weak electrostatic interactions. Complex 2-Yb is isostructural to previously reported [(BDI*)MgNa/CaN’’2] (2-Ca). Weak Mg0-M2+ (M = Ca, Yb) bonding is supported by facile exchange reactions with (SrN’’2)2 leading to formation of [(BDI*)MgNa/SrN’’2] (2-Sr). Energy decomposition analysis (EDA-NOCV) confirmed major electrostatic Mg0-M2+ bonding (63–66%), while orbital interactions are less significant (19–20%). Both, 2-Ca and 2-Yb show considerable Mg0 → M2+ electron transfer leading to oxidation-state ambiguities (Mg0-M+II ↔ Mg+I-M+I). Effective-Oxidation-State calculations assign the following metal oxidation states, Na+I-Mg0-M+II, with a reliability index of 69–72%. Despite structural and electronic similarities, there are differences in reactivities. Complex 2-Ca reacts only as a 2-electron reducing agent whereas 2-Yb can deliver up to three electrons. This study includes reactions with N2O, TEMPO, and Me3PX (X = O, S, Se). While only 2-Ca could be converted in a complex with incorporated O2–, complexes with S2– or Se2– have been isolated for both.