Synthesis, Structure, and Reactivity of a Magnesium(0) Complex with a Polarized Mg^δ-─Ca^δ+ Bond

Despite numerous examples of Mg─Mg bound complexes, complexes with unsupported two-center-two-electron Mg─Ca bonds so far always eluded their isolation. This is due to Mg^δ¯-Ca^δ+ polarization and their inherently very high reactivity resulting in oxidative addition to C─H bonds in either the ligand or solvent. Using a combination of bulky β-diketiminate (BDI*) and amidinate (Am*) ligands, we now present (BDI*)Mg-Ca(Am*); BDI* = HC[C(tBu)N(DIPeP)]₂, DIPeP = 2,6-Et₂C-phenyl; Am* = tBuC(N-DIPP)₂, DIPP = 2,6-iPr-phenyl. The crystal structure shows a Mg─Ca bond of 3.298 Å and Mg/Ca NPA charges of +0.66/+1.28. This reflects considerable Mg^δ¯-Ca^δ+ polarization and is in line with the formal oxidation states Mg⁰-Ca^II. Effective-Oxidation-State (EOS) analysis confirms these formal oxidation states, but a low reliability index of 69.8% indicates that these are not fully representative for the electron distribution. Atoms-In-Molecules (AIM) shows a bond-critical-point on the Mg-Ca axis but a non-nuclear-attractor (NNA) was not observed. EDA-NOCV using two different fragmentations, (BDI*)Mg-/(Am*)Ca⁺ or (BDI*)Mg^●/(Am*)Ca^●, shows that the complex is best described as a combination of neutral radicals. The Mg⁰-Ca^II/Mg^I-Ca^I ambiguity results in a highly reactive Mg^δ¯─Ca^δ+ bond. This is confirmed by its reaction with CyN═C═NCy and Ph₂C═CH₂ showing the electrophilic nature of Ca and nucleophilic reactivity for Mg.