Storing a saturated THF/MTBE solution of 5 at -40 °C yielded single crystals suitable for analysis X-ray diffraction (Figure ). The solid state-structure of 5 confirms the presence of a Ge and a Mg centre bound to the dipp NBA ligand. The solid-state structure also corroborates the formation of the bicyclo[2,2,1]heptane motif, consistent with our spectroscopic observations. The Ge-Mg distance of 2.9115(15) Å lies well within the sum of their van der Waals radii (4.80 Å) and outside of the sum of their covalent radii (2.62 Å). Additionally, the distance is significantly longer compared to other reported Ge-Mg distances. These observations are consistent with a dative Ge-Mg bond being present rather than a covalent bond (see below). The geometry at the Ge centre is distorted tetrahedral and the geometry around the Mg centre is distorted octahedral, with three meridionally binding THF ligands completing the coordination sphere. The C9-C10 bond length of 1.380(6) Å is consistent with a double bond, indicative of the two-electron reduction of the binding pocket coordinated to Mg(II). On the Ge side of the complex, the C1-C2 (1.511(6) Å) and C35-C36 (1.342(6) Å) bond lengths are consistent with single and double bond character, respectively. The addition of one of the unsaturated C-C bonds of the acetylene moiety over the NHGe motif can best be described as a formal [4+2] cycloaddition or 'Metallo-Diels-Alder' reaction of p-tolylacetylene with the NHGe motif from resonance structure C (Scheme 4). Figure : Displacement ellipsoid plot of 5 at 50% probability. Hydrogen atoms and minor disorder components are omitted for clarity. The dipp substituents and the carbon backbones of the THF molecules are depicted as wireframe for clarity.