Structure and bonding of a radium coordination compound in the solid state

The structure and bonding of radium (Ra) is poorly understood because of challenges arising from its scarcity and radioactivity. Here we report the synthesis of a molecular Ra²⁺ complex using ²²⁶Ra and the organic ligand dibenzo-30-crown-10, and its characterization in the solid state by single-crystal X-ray diffraction. The crystal structure of the Ra²⁺ complex shows an 11-coordinate arrangement comprising the 10 donor O atoms of dibenzo-30-crown-10 and that of a bound water molecule. Under identical crystallization conditions, barium (Ba²⁺) yielded a 10-coordinate 'Pac-Man'-shaped structure lacking water. Furthermore, the bond distance between the Ra centre and the O atom of the coordinated water is substantially longer than would be predicted from the ionic radius of Ra²⁺ and by analogy with Ba²⁺, supporting greater water lability in Ra²⁺ complexes than in their Ba²⁺ counterparts. Barium often serves as a non-radioactive surrogate for radium, but our findings show that Ra²⁺ chemistry cannot always be predicted using Ba²⁺.