Thallium cyano complexes

Nitrosalicylhydrazide, 2778 Scandium 3-nitrobenzoate, 3816 Silver osmate, 0034 Thallium bromate, 0260 Thallium(I) methanediazoate, 0458 Thallium(I) 2- or 4-nitrophenoxide, 2187 Thallium acz-phenylnitromethanide, 2723 See also METAL AZIDES, METAL CYANIDES (AND CYANO COMPLEXES), /V-MKTAL DERIVATIVES... 

Bacillus ferredoxins, cysteine residues, 38 246 Bacillus thermoproteolyticus ferredoxins, chain topology, 38 244, 246 Back donation and bonding, 16 92, 93 thallium(III) cyano complexes, 43 17-18 (d-d)n Back donation, 33 335 Bacteria... 

The salt T1CN can be prepared by the reaction of thallium(I) acetate and KCN in aqueous solution,279 but there is no evidence for the formation of cyano complexes in aqueous solution.280... 

These spectral features and other evidence presented in this paper (97) leave no doubt that the Tl(III) cyano complexes exist, that they have the stated composition, and that they are extremely strong and kinetically inert. In fact, the cyano complexes are stronger than any other known monodentate complexes of thallium(III). The only possible known competitor as a ligand, the iodide ion, forms the complex TII4 with the overall stability constant log = 35.7 (99), i.e., several orders of magnitude lower than that of T1(CN)4" (see Table III). The distribution of thallium among the various Tl(CN), "-species is shown in Fig. 5. Stepwise stability constants of MX complexes often decrease with increasing n because of statistical, steric, and coulombic factors... 

There is some evidence that back-donation plays an important role in cyano complexes (149). For the isoelectronic Au(I), Hg(II), and Tl(III) ions, the back-donation should be most efficient (and hence the complexes should be strongest) for gold and least efficient for thallium because of the increasing charge on the metal ion. Thus, if back-donation is a major effect, the stability constants for the cyano complexes should decrease in the order Au > Hg > Tl. Unfortunately, only one stability constant, namely /32, is (approximately) known for gold(I), but there is no doubt that the 2-values follow the predicted trend log 82 = 39 (for Au) > 32.7 (for Hg) > 26.5 (for Tl) (97, 150,151). 

Fig. 11. Proposed structures and the determined interatomic distances (in angstroms) for thallium(III) cyano complexes in aqueous solution. The water oxygens are dark, the C and N atoms are shaded. Reprinted fi-om Blixt et al. (145). Copyright 1995 American Chemical Society.

Relatively little work has been done on the redox reaction between thallium and halide/pseudohalide ions (75, 93, 97,110, 328-330). Let us consider the qualitative order of stability of thallium(III) in the form of TlXp " complexes, where X = Cl, Br, I, SCN, CN, Thallium(III) forms strong complexes with all these ligands on the other hand, it can oxidize the X ions to X2. It is well known that the thallium(III) chloride complexes are perfectly stable for an indefinite period of time. The corresponding bromide complexes are usually stable, but at low Br/Tl ratios Tl(III) can be reduced by Br the reduction is easily prevented by adding excess of bromine. The iodo and thiocyanato complexes are approximately equally unstable toward redox reaction Tl(III) is rapidly reduced by the anion.Finally, the cyano complexes... 

Recently, the formation, structure, equilibrium, and kinetics of a family of cyano compounds containing a direct and not supported by ligands platinum-thallium metal-metal bond have been reported. The complexes are synthesized according to the reaction ... 

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