Revised Tl(I)-O bond valence parameters and the structures of thallous dichromate and thallous uranyl phosphate hydrate

Thallium(I) has a 6s(2) non-bonding pair of valence-shell electrons, and shows a wide range of coordination environments and mean bond distances to oxygen. Conventional bond valence parameters with B = 0.37 Angstrom are not suitable for modelling the soft acid - hard base Tl(I)-O interactions. Revised bond valence parameters have been derived for these bonds using structural data taken from the Inorganic Crystal Structure Database and the Cambridge Structural Database for 130 Tl(I)-O-n polyhedra. Both bond valence parameters, R-0 and B, were refined by minimizing the sum of the squared differences between the formal oxidation state and the experimental bond valence sums of the polyhedra. The recommended values for TI(I)-O bonds are R-0 = 1.927 Angstrom, B = 0.50 Angstrom, and for the assembled set of TI-O polyhedra yield an average sum of 0.99 vu with a standard deviation of 0.12vu. The effects of the revised TI-O bond valence parameters upon structures containing different types of polyhedra of higher bond valence are examined with respect to the crystal structures of thallous dichromate and thallous uranyl phosphate hydrate. Crystallographic data - Tl2Cr2O7: triclinic, PI, a = 7.3150(15) Angstrom, b = 7.4006(15) Angstrom, c = 7.6673(16) Angstrom, a = 109.941(4)degrees, beta = 90.458(4)degrees, gamma = 108.067(4)', R1 = 5.9%; Tl-2(UO2)[(UO2)(PO4)](4)(H2O)(2): monoclinic, Cm, a 12.9798(g) Angstrom, b = 15.1639(11) Angstrom, c = 9.3384(6) Angstrom, beta = 312.310(1), R1 = 3.6%.