Comparison of MgYb and Mg-Ca bonding: structures, electronic analyses and reactivities

Reaction of [(BDI*)MgNa]₂ with (Am*YbI)₂ in methylcyclohexane led to the precipitation of NaI and quantitative formation of (BDI*)Mg-Yb(Am*) (1-Yb); BDI* = HC[C(tBu)N(DIPeP)]₂, DIPeP = 2,6-Et₂CH-phenyl; Am* = tBuC(N-DIPP)₂, DIPP = 2,6-iPr-phenyl. Complex 1-Yb features a long unsupported 2c-2e Mg-Yb bond of 3.302 Å and crystallizes isomorphous to (BDI*)Mg-Ca(Am*) (1-Ca). The formal Mg⁰-Yb^II oxidation states in 1-Yb are confirmed by effective-oxidation-state analysis but the low reliability index of 68.0% indicates that this description is not fully representative of the electron distribution. Analysis of the electron density shows significant Mg⁰ → Yb^II electron transfer and therefore the complex has partial Mg^I-Yb^I character. EDA-NOCV analysis suggests that bonding in 1-Yb is best pictured as a combination of neutral open-shell radicals (BDI*)Mg^˙/(Am*)Yb^˙. Accordingly, (BDI*)Mg acts as an X-type ligand for Yb(Am*). The long, polarized Mg ^δ--Yb ^δ+ bond in 1-Yb is highly reactive. The thermal decomposition and reactivity of 1-Yb with N₂O, NH₃, 1,1-diphenylethylene, trans-stilbene and anthracene have been studied. Comparison with the reactivity of 1-Ca shows strong similarities, but also important differences. Most reactants oxidize the Mg⁰ centre in 1-Yb and 1-Ca and produce similar complexes that generally crystallize isomorphous. However, N₂O can also oxidize the Yb^II centre, resulting in paramagnetic Yb^III products. In all cases, the Mg-Yb and Mg-Ca bonds in 1-Yb and 1-Ca are considerably more reactive than the unpolarized Mg-Mg bond in complexes of type (BDI)Mg-Mg(BDI).