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 O^2-, complexes with S^2- or Se^2- have been isolated for both.