Radium Revisited: Revitalization of the Coordination Chemistry of Nature's Largest +2 Cation.

The crystallization, single crystal structure, and Raman spectroscopy of Ra(NO 3 ) 2 have been investigated by experimentation and theory, which represent the first pure radium compound characterized by single crystal X-ray diffraction. The Ra 2+ centers are bound by six chelating nitrate anions to form an anticuboctahedral geometry. The Raman spectrum acquired from a single crystal of Ra(NO 3 ) 2 generally occurs at a lower frequency than found in Ba(NO 3 ) 2 , as expected. Computational studies on Ra(NO 3 ) 2 provide an estimation of the bond orders via Wiberg bond indices and indicate that Ra-O interactions are weak with values of 0.025 and 0.026 for Ra-O bonds. Inspection of natural bond orbitals and natural localized molecular orbitals suggest negligible orbital mixing. However, second-order perturbation interactions show that donation from the lone pairs of the nitrate oxygen atoms to the 7s orbitals of Ra 2+ stabilizes each Ra-O interaction by ca . 5 kcal mol -1 .