Structure of Thallium(III) Chloride, Bromide, and Cyanide Complexes in Aqueous Solution

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Abstract

The structures of the hydrated thallium(III) halide and pseudohalide complexes, [TlX(n)(OH2)(m)]((3-n)+), X = Cl, Br, CN, in aqueous solution have been studied by a combination of X-ray absorption fine structure spectroscopy (XAFS), large-angle X-ray scattering (LAXS), and vibrational spectroscopic (Raman and IR) techniques including far-infrared studies of aqueous solutions and some solid phases with known structures. The vibrational Tl-X frequencies of all complexes are reported, force constants are calculated using normal coordinate analysis, and assignments are given. The structural results are consistent with octahedral six-coordination for the cationic complexes Tl(OH2)(6)(3+), TlX(OH2)(5)(2+), and trans-TlX(2)(OH2)(4+). The coordination geometry changes to trigonal bipyramidal for the neutral TlBr3(OH2)(2) complex and possibly also for TlC3(OH2)(2). The TlX(4)(-) complexes are all tetrahedral. Higher chloride complexes, TlCl5(OH2)(2-) and TlCl63-, are formed and have again Octahedral coordination geometry. For the first and second halide complexes, TlX(OH2)(5)(2+) and TlX(2)(OH2)(4+), no lengthening was found of the Tl-X bonds, with Tl-Br distances of 2.50(2) and 2.49(2) Angstrom, respectively, and Tl-Cl distances of 2.37(2) Angstrom for both complexes. The mean Tl-O bond distances increase slightly, approximate to 0.04 Angstrom, from that of the Tl(OH2)(6)(3+) ion, at the formation of the first thallium(m) halide complexes. A further, more pronounced lengthening of about 0.1 Angstrom occurs when the second complex forms, and it can be related to the relatively high bond strength in the trans-XTlX entity, which also is manifested through the Tl-X stretching force constants. For the recently established Tl-3(CN)(n)((3-n)+) complexes with no previously available structural information, the Tl-C distances were determined to be 2.11(2), 2.15(2), and 2.19(2) Angstrom for n = 2, 3, and 4, respectively. The Tl(CN)(2)+ complex has a linear structure, and the Tl(CN)(4)(-) complex is tetrahedral with the CN- ligands linearly coordinated. The lower complexes (n = 1-3) are hydrated, although the coordination numbers could not be unambiguously determined. A well-defined second coordination sphere corresponding to at least eight water molecules at a Tl-O-II distance of approximate to 4.3 Angstrom was found around the second complex, probably trans-Tl(CN)(2)(OH2)(4+). The third cyano complex is probably pseudotetrahedral, Tl(CN)(3)(OH2). The bonding in the hexahydrated Tl3+ and Hg2+ ions is discussed, and differences in the mean M-O bond lengths, determined by the LAXS and EXAFS techniques, are interpreted as being due to an occurrence of two different sets of M-O distances in the first hydration shell.

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