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

The synthesis, structure, and Raman spectrumof Ra-(NO3)(2) have been investigated by experimentationandtheory. Single crystal X-ray diffraction analysis reveals that radiumnitrate is isomorphous with Ba-(NO3)(2) and crystallizesin the cubic space group Pa 3 (No. 205) with a = 8.2268(1) angstrom at 100 K. Ba-(NO3)(2) has been further characterized with the useof Raman spectroscopy. The crystallization, single crystal structure, and Ramanspectroscopyof Ra-(NO3)(2) have been investigated by experimentationand theory, which represent the first pure radium compound characterizedby single crystal X-ray diffraction. The Ra2+ centers arebound by six chelating nitrate anions to form an anticuboctahedralgeometry. The Raman spectrum acquired from a single crystal of Ra-(NO3)(2) generally occurs at a lower frequency than foundin Ba-(NO3)(2), as expected. Computational studieson Ra-(NO3)(2) provide an estimation of the bondorders via Wiberg bond indices and indicate that Ra-O interactionsare weak with values of 0.025 and 0.026 for Ra-O bonds. Inspectionof natural bond orbitals and natural localized molecular orbitalssuggest negligible orbital mixing. However, second-order perturbationinteractions show that donation from the lone pairs of the nitrateoxygen atoms to the 7s orbitals of Ra2+ stabilizes eachRa-O interaction by ca. 5 kcal mol(-1).

Automated summary

The synthesis, structure, and Raman spectrum of Ra-(NO3)(2) were investigated by experimentation and theory. Single crystal X-ray diffraction analysis showed that radium nitrate is isomorphous with Ba-(NO3)(2) and crystallizes in the cubic space group Pa 3 with a = 8.2268(1) angstrom at 100K. Ra-(NO3)(2) represents the first pure radium compound characterized by single crystal X-ray diffraction. The Raman spectrum of Ra-(NO3)(2) generally occurs at a lower frequency compared to Ba-(NO3)(2), as expected. Computational studies provided an estimation of bond orders and indicated weak Ra-O interactions.