Synthesis, Structure, and Spectroscopy of Rare Earth Hypophosphites. 2. Uranyl Hypophosphite Monohydrate and Uranyl Hypophosphite-Hypophosphorous Acid (1/1).

The new compounds UO(2)(H(2)PO(2))(2).H(2)O (1) and UO(2)(H(2)PO(2))(2).H(3)PO(2) (2) crystallize in the space groups P2(1)/n and P2(1)2(1)2(1), respectively, each with Z = 4. For 1, a = 7.686(2) Å, b = 9.275(2) Å, c = 11.027(2) Å, alpha = 90 degrees, beta = 92.32(3) degrees, gamma = 90 degrees; for 2, a = 7.1572(4) Å, b = 7.2363(6) Å, c = 17.554(2) Å, alpha = beta = gamma = 90 degrees. Each uranium atom, situated at a general position, has a distorted pentagonal bipyramidal coordination geometry, and U-O-P-O-U bridges form differently oriented chains in 1 and a three-dimensional network structure in 2. H(3)PO(2) serves as a terminal monodentate ligand in 2. The infrared and Raman spectra of 1 and 2 show (i) characteristic uranyl modes of vibration; (ii) partially resolved unit cell group modes of hypophosphite anions; and (iii) bands due to water and H(3)PO(2), respectively. The C(1) site symmetry in these compounds makes their spectral properties useful in evaluating the presence of additional electronic transitions not identified in the analyses of higher symmetry uranyl compounds. In the region between 20 000 and 29 000 cm(-)(1), 12 electronic transitions have been located and assigned. The luminescence from both compounds is weak due to competing nonradiative processes, and the major differences from that of UO(2)(2+) at centrosymmetric sites arise from the electric dipole intensity enhancement of the zero-phonon line and the appearance of progressions in the uranyl antisymmetric stretching mode. Water and hypophosphite modes are identified in the luminescence of 1 and 2, respectively. The anharmonicities of uranyl stretching modes in 1 are compared with those in other uranyl systems. The Franck-Condon analysis of the absorption and emission spectra of 1 show a U-O bond length increase of 5 +/- 1 pm on excitation from to sigma(u)delta(u), compared with values ranging from 4 to 9 pm for other uranyl systems.