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

The new compounds UO2(H2PO2)2·H2O (1) and UO2(H2PO2)2·H3PO2 (2) crystallize in the space groups P21/n
and P212121, respectively, each with Z = 4. For 1, a = 7.686(2) Å, b = 9.275(2) Å, c = 11.027(2) Å, α = 90°,
β = 92.32(3)°, γ = 90°; for 2, a = 7.1572(4) Å, b = 7.2363(6) Å, c = 17.554(2) Å, α = β = γ = 90°. 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.
H3PO2 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 H3PO2, respectively. The C1 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 UO22+ 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 σuδu, compared with values ranging from 4 to 9 pm for other uranyl systems.