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Inorg. Chem. 1997, 36, 5900-5908
Equilibrium Dynamics in the Thallium(III)-Cyanide System in Aqueous Solution Istva´ n Ba´ nyai,*,† Julius Glaser,*,‡ and Judit Losonczi†,§ Department of Inorganic Chemistry, The Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden, and Department of Physical Chemistry, Lajos Kossuth University (KLTE), H-4010 Debrecen Pf. 7, Hungary ReceiVed March 28, 1997X
Ligand exchange reactions of thallium(III) cyano complexes, Tl(CN)n3-n, have been systematically studied in aqueous solution containing 4 M ionic medium {[ClO4-]tot ) 4 M, [Na+]tot ) 1 M, [Li+]tot + [H+]tot ) 3 M}, at 25 °C, using 205Tl and 13C NMR one-dimensional inversion transfer techniques. Rate constants for all dominating exchange pathways were determined and compared to the previously studied thallium(III) halide complexes. Also in the case of cyanide ligands the ligand exchange is dominated by the rare type of reactions occurring Via a direct encounter of two complexes (self-exchange reactions), e.g. Tl(CN)3 + Tl(CN)2+ h Tl(CN)2+ + Tl(CN)3 (k32, k23) or Tl(CN)2+ + Tl(CN)4- h 2Tl(CN)3 (k24, k33). The determined cyanide exchange rate constants between complexes Tl(CN)m3-m and Tl(CN)n3-n, k rds mn, for the rate-determining step have all similar values, about 1001000 s-1, that are 5 orders of magnitude smaller than for the corresponding halide exchange processes. This indicates the presence of a common rate-determining step for the self-exchange reactions of the cyanide ligand, proposed to be the breaking of the thermodynamically very stable Tl-CN bond. This is in contrast to the Tl(III)-halide systems, where the breaking of the Tl-OH2 bond was proposed to determine the reaction rate. The second type of cyanide exchange, namely anation, has been found only in two cases: Tl(CN)2+ + CN- h Tl(CN)3 (k ′23, k ′32); and Tl(CN)3 + CN- h Tl(CN)4- (k ′34, k ′43). These reactions are very fast, k ′mn ∼ 109 M-1 s-1, and are proposed to proceed through an associative interchange mechanism, where the rate-determining step is a water dissociation mediated by the incoming ligand, i.e. similarly as for the corresponding halide complexes. The third type of cyanide exchange reactions was possible to study due to the presence of an NMR-active nucleus (13C) in the ligand. Only the following ligand substitution reaction was observed: Tl(*CN)2+ + HCN h Tl(CN)2+ + H*CN (k *2,HCN, k *HCN,2). The reason for the dominant role of the self-exchange reactions is the very low concentration of free CN- and the inertness of the HCN species in the ligand exchange reactions. The obtained dynamic information is discussed and compared to the corresponding data for the thallium(III) halide complexes.
Thallium(III) forms very strong complexes with halide ions.1-6 Recently, the formation of even stronger complexes between Tl(III) and cyanide, Tl(CN)n3-n, n ) 1-4, was established and their stability constants were determined.7 The structure of the halide complexes was studied both in solid state and in solution by means of IR,8 Raman,9-11 X-ray diffraction,12,13 and 205Tl NMR spectroscopy.6 Very recently, a detailed multimethod study was published on the structure, symmetry, and coordination numbers of Tl(III) chloro, bromo, and cyano complexes in aqueous solution.14 In the first cyano complex Tl(III) is six-coordinated, TlCN(OH2)52+. In the †
Lajos Kossuth University. The Royal Institute of Technology. On leave at the Department of Chemistry, Yale University, 225 Prospect St., New Haven, CT 06511. X Abstract published in AdVance ACS Abstracts, November 1, 1997. (1) Lee, A. G. The Chemistry of Thallium; Elsevier: Amsterdam, 1971; p 14. (2) Busev, A. I.; Tiptsova, V. G.; Sokolova, T. A. Vestnik MoskoV. UniV. Khim. 1960, 11, 6. (3) Ahrland, S.; Grenthe, I.; Johansson, L.; Nore´n, B. Acta Chem. Scand. 1963, 17, 1567. (4) Ahrland, S.; Johansson, L. Acta Chem. Scand. 1964, 18, 2125. (5) Leden, I.; Ryhl, T. Acta Chem. Scand. 1964, 1196. (6) Glaser, J.; Henriksson, U. J. Am. Chem. Soc. 1981, 103, 6642. (7) Blixt, J.; Gyo¨ri, B.; Glaser, J. J. Am. Chem. Soc. 1989, 111, 7784. (8) Davies, E. D.; Long, D. A. J. Chem. Soc. 1968, 2050. (9) Spiro, T. G. Inorg. Chem. 1965, 4, 731. (10) Spiro, T. G. Inorg. Chem. 1965, 4, 1290-3. (11) Spiro, T. G. Inorg. Chem. 1967, 6, 569. (12) Glaser, J.; Johansson, G. Acta Chem. Scand. 1982, A36, 125. (13) Glaser, J. Acta Chem. Scand. 1982, A36, 451. (14) Blixt, J.; Glaser, J.; Mink, J.; Persson, I.; Persson, P.; Sandstro¨m, M. J. Am. Chem. Soc. 1995, 117, 5089. ‡ §
S0020-1669(97)00366-2 CCC: $14.00
second complex, trans-TlCN2(OH2)4+, thallium retains the sixcoordination; the Tl-CN bond length is almost the same as in the first complex, 2.11 Å, and Tl-O is 2.42 Å, that is 0.2 Å longer than in the hydrated thallium(3+) ion, Tl(OH2)63+. Upon formation of the third complex, Tl(CN)3(OH2), the coordination number decreases to 4 and the symmetry is C3V with Tl-C 2.15 Å and Tl-O 2.42 Å. The fourth complex is tetrahedral with the Tl-C bond length equal to 2.19 Å. The dynamics of aluminum group elements has been scarcely studied because of the difficulty to monitor the reactions. The solvent exchange was investigated for aluminum, gallium, and indium.15-17 The mechanism of the solvent exchange is Id for Al3+ and Ga3+, but it is associative for In3+. The complex formation reactions of Al3+ occur by Id mechanism,18,19 whereas an associatively activated mechanism was proposed for gallium(III) and is also possible for indium(III).19 The knowledge about dynamic properties of thallium(III) is even more limited. The kinetics of complex formation with semixylenol orange and 4-(2-pyridylazo)resorcinol were studied by the stopped-flow method.20,21 The complex Tl(OH)2+ was found to be the active (15) Ammann, C.; Moore, P.; Merbach, A. E.; McAteer, C. H. HelV. Chim. Acta 1980, 63, 268. (16) Merbach, A. E. Pure Appl. Chem. 1982, 54, 1479. (17) van Eldik, R. In Inorganic High Pressure Chemistry Kinetics and Mechanism; van Eldik, R., Ed.; Elsevier: Amsterdam-Oxford-New York-Tokyo, 1986; p 69. (18) Hugi-Cleary, D.; Helm, L.; Merbach, A. E. HelV. Chim. Acta 1985, 68, 545. (19) Miceli, J.; Stuehr, J. J. Am. Chem. Soc. 1968, 90, 6967. (20) Kawai, Y.; Takahashi, T.; Hayashi, K.; Imamura, T.; Nakayama, H.; Fujimoto, M. Bull. Chem. Soc. Jpn. 1972, 45, 1417.
© 1997 American Chemical Society
The Thallium(III)-Cyanide System
Inorganic Chemistry, Vol. 36, No. 25, 1997 5901
Table 1. Kinetic Data for Tl(III) Cyano and Halo Complexes bromide24 chloride23
cyanide -1 k rds mn/s
kmn/M-1 s-1 k01 k02 k12 k23 k34 k24 k ′12 k ′23 k ′34
-1 k rds mn/s
