Barium Azacryptand Sodide, the First Alkalide with an Alkaline Earth Cation, Also Contains a Novel Dimer, (Na<sub>2</sub>)<sup>2-</sup>

The first barium sodide, with stoichiometry Ba<sup>2+</sup>(H<sub>5</sub>Azacryptand[2.2.2]<sup>-</sup>)Na<sup>-</sup>·2MeNH<sub>2</sub>, was synthesized by the reaction of Ba, Na, and H<sub>6</sub>Azacryptand[2.2.2] in NH<sub>3</sub>−MeNH<sub>2</sub> solution. It was characterized by X-ray crystallography, <sup>23</sup>Na MAS NMR, hydrogen evolution, DSC, optical spectroscopy, and magnetic susceptibility. This is the first sodide in which the sodium anions form (Na<sub>2</sub>)<sup>2-</sup> dimers. Previous theoretical predictions were verified by a calculation of the potential energy curve for the dimer in the field of the surrounding charges, whose positions were determined from the crystal structure.