A comparison of the interatomic distances of Ra1 versus the conformationally similar Ba2 and Ba3 structures also reveals marked differences between Ra 2+ and Ba 2+ in the context of bonding. The M-Oether bond distances in Ra1 range from 2.907(2) Å to 3.086(2) Å, which are slightly longer than the bond distances in Ba2 and Ba3 (2.789-3.164 Å). These longer bond distances are likely explained by the larger ionic radius of Ra 2+ in comparison to Ba 2+ . More pronounced, however, is the difference in the interatomic distances between the metal center and the coordinated water molecule in these structures. Specifically, the Ba-Owater bond distances in Ba2 and Ba3 are 2.763 Å and 2.780 Å, respectively. By contrast, the Ra-Owater bond distance in Ra1 is approximately 0.14 Å longer (2.913 Å) (see Table for crystallographic data). This bond distance in Ra1 is significantly longer than expected considering the 0.06 Å difference in ionic radii between Ba 2+ and Ra 2+ , which suggests that Ra 2+ binds water more weakly than does Ba 2+ . Our results are consistent with those reported recently for the Ra-aquo ion, for which the average Ra-Owater bond was estimated to be 2.86 ± 0.06 Å. Furthermore, when examining other alkaline earth crown ether complexes possessing coordinated water molecules, there appears to be a trend of increasing M 2+ -Owater bond distances and decreasing M 2+ -Owater binding energies with increasing ionic radii. These trends in the alkaline earth series, coupled with the Ra-Owater bond length reported in this work, support the concept of greater lability of bound water molecules in Ra 2+ compounds compared to Ba 2+ compounds.