STRUCTURE OF THALLIUM(III) CHLORIDE, BROMIDE, AND CYANIDE COMPLEXES IN AQUEOUS-SOLUTION

The structures of the hydrated thallium(III) halide and pseudohalide c omplexes, [TlX(n)(OH2)(m)]((3-n)+), X = Cl, Br, CN, in aqueous solutio n have been studied by a combination of X-ray absorption fine structur e spectroscopy (XAFS), large-angle X-ray scattering (LAXS), and vibrat ional spectroscopic (Raman and IR) techniques including far-infrared s tudies of aqueous solutions and some solid phases with known structure s. The vibrational Tl-X frequencies of all complexes are reported, for ce constants are calculated using normal coordinate analysis, and assi gnments are given. The structural results are consistent with octahedr al 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 pos sibly also for TlC3(OH2)(2). The TlX(4)(-) complexes are all tetrahedr al. Higher chloride complexes, TlCl5(OH2)(2-) and TlCl63-, are formed and have again Octahedral coordination geometry. For the first and sec ond halide complexes, TlX(OH2)(5)(2+) and TlX(2)(OH2)(4+), no lengthen ing 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, a pproximate to 0.04 Angstrom, from that of the Tl(OH2)(6)(3+) ion, at t he formation of the first thallium(m) halide complexes. A further, mor e pronounced lengthening of about 0.1 Angstrom occurs when the second complex forms, and it can be related to the relatively high bond stren gth 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 wi th the CN- ligands linearly coordinated. The lower complexes (n = 1-3) are hydrated, although the coordination numbers could not be unambigu ously determined. A well-defined second coordination sphere correspond ing to at least eight water molecules at a Tl-O-II distance of approxi mate to 4.3 Angstrom was found around the second complex, probably tra ns-Tl(CN)(2)(OH2)(4+). The third cyano complex is probably pseudotetra hedral, Tl(CN)(3)(OH2). The bonding in the hexahydrated Tl3+ and Hg2ions is discussed, and differences in the mean M-O bond lengths, deter mined by the LAXS and EXAFS techniques, are interpreted as being due t o an occurrence of two different sets of M-O distances in the first hy dration shell.