The reaction of a sterically shielded Mg hydride complex, bearing a bulky β‐diketiminato (BDI) ligand, with CO 2 leads to the formation of polynuclear complexes. CO 2 not only inserts into the metal hydride bond yielding formate ions, but also adds onto the γ‐C of the BDI ligand. This reactivity gives rise to the formation of soluble tri‐ and tetranuclear structures in THF, leading to a hexameric structure associated with a change in the coordination mode of the BDI ligand at 60 °C in THF within four days. The insoluble hexameric complex was isolated in high reproducible yield from the solution and characterized by X‐ray diffraction analysis. The reaction affording the hexameric complex was shown to be completed within 1 h at room temperature in toluene. Similar reactions of CO 2 with a BDI calcium hydride complex proceeds less cleanly and a pure product could not be isolated.