<i>s</i>-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 <i>p</i>- or <i>d</i>-block metals. Exemplified by a Mg-Yb complex, we present synthesis and extensive computational analyses of the first <i>s</i>-block metal-Ln bond. Key to its formation is the unique Mg⁰ complex [(BDI*)MgNa]₂ (<b>1</b>) which can be viewed as a dimer consisting of two magnesyl anions, (BDI*)Mg^-, bridged by two Na⁺ cations (BDI* = HC[C(<i>t</i>Bu)N-DIPeP]₂, DIPeP = 2,6-CHEt₂-phenyl). Reaction of dimeric <b>1</b> with (YbN''₂)₂ (N'' = N(SiMe₃)₂) quantitatively gave the mixed complex [(BDI*)MgNa/YbN''₂] (<b>2</b>-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 <b>2</b>-Yb is isostructural to previously reported [(BDI*)MgNa/CaN''₂] (<b>2</b>-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] (<b>2</b>-Sr). Energy decomposition analysis (EDA-NOCV) confirmed major electrostatic Mg⁰-M²⁺ bonding (63-66%), while orbital interactions are less significant (19-20%). Both, <b>2</b>-Ca and <b>2</b>-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 <b>2</b>-Ca reacts only as a 2-electron reducing agent whereas <b>2</b>-Yb can deliver up to three electrons. This study includes reactions with N₂O, TEMPO, and Me₃PX (X = O, S, Se). While only <b>2</b>-Ca could be converted in a complex with incorporated O^2-, complexes with S^2- or Se^2- have been isolated for both.