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In contrast to what is usually assumed, we have found and proved that thallium(III) forms very strong cyanide complexes in aqueous solution. We have investigated this system using 205T1,13C, and 14N NMR and potentiometry and established the existence of four thallium(III)-cyanide complexes of the composition T1(CN)"3"", = 1-4. We have measured their chemical shifts and spin-spin coupling constants and determined their formation constants at 25 °C in dilute (0.05 M) aqueous solution in the ionic medium ([Na+] = 1 M, [Li+] + [H+] = 3 M, [C104"] = 4 M). The overall formation constants are log ß = 13.2 (1), log ß2 = 26.5 (2), log ß2 = 35.2 (2), and log = 42.6 (2). We have also determined the stability constant for HCN in the same ionic medium, log K" = 10.11 (5). One-bond spin-spin coupling constants between thallium and carbon, 1/(205T1-13C), for the mono-and dicyano complexes of thallium(III) are 14636 and 13749 Hz, respectively, and appear to be the largest known coupling constants between these nuclei. The stability of the T1(CN)"3"" complexes is discussed in terms of their kinetic inertness.

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Thallium(II1) in aqueous solution is known to be a strong o~i d a n t .~J The thiocyanate ion forms strong complexes with several metal ions, including d10 ions. It is generally assumed that pseudohalide ions, with their reducing properties, cannot exist in solutions containing thallium(III), since they undergo an immediate oxidation. For thiocyanate ion, the following reaction was found to occur:4 (1) About 20 years ago, the kinetics of this redox reaction was studied by means of polarography.5 On the basis of the dependence of the rate of the reaction on the concentrations of thallium(II1) and thiocyanate, Treindl and Fico suggested5 that the reaction proceeds through a short-lived intermediate with the composition T1-(SCN)TP+. Species like T12(SCN)n6n were also proposed to exist. Recently, Gupta et a1.6 performed a similar kinetic investigation and proposed the existence of the complexes Tl(SCN)2+, T1-(SCN)2+, T12(SCN)5+, T12(SCN)24+ or (TI(SCN)}24+, and HTI(SCN);+. These proposals are surprising, since polynuclear thallium(II1) species have never been reported in aqueous solut i~n .~.~ Having in our hands the new powerful experimental technique of multinuclear NMR spectrampy and encouraged by our recent discovery that, contrary to the accepted ~p i n i o n ,~ thallium( 111) cyanide complexes in aqueous solution exist and are extremely strong and stable,' we wanted to see if we could find and characterize the suggested5v6 (or any other) reaction intermediates in the course of the redox reaction between Tl(II1) and thiocyanate.The previous authors5 used TlC13 as a source of thallium(II1).Chloride ions complicate the chemistry of the system, since their presence may cause formation of several typm of complexes, such as TIC1,' -n*8 Tl(CN),Cl,'-"-n*9 etc. In order to simplify the chemical system, our starting solutions contained only Tl(C104)3 in HC104. Experimental SectionMaterials. The stock solution of 1 M TI(CI0,)3 was obtained by anodic oxidation of TIC104.10 I3C-enriched NaSCN was prepared by mixing, grinding, and gently fusing 1 mmol of I3C-enriched NaCN (Cambridge Isotope Laboratories, Woburn, MA) with 1 mmol of sulfur and 0.25 mmol of NaSCN. After continued heating in order to remove excess sulfur, the melt was cooled and dissolved in water. This procedure resulted in NaSCN enriched 75% in "C. l5N-enriched NaSCN (99%) was purchased from MSD Isotopes, Montreal. The stock solution of thallium(II1) was then mixed at 0 OC with the appropriate stock solution of sodium thiocyanate (0.040 M) and 3 M HCIO,, giving the following concentrations at the beginning of the reaction: [TI(III)] = 0.17 M, [SCN-] = 0.025 M, and [H+] = 0.52 M. The ionic strength was kept approximately constant by means of LiCIO, ([Li+] + [H' ] -3 M).AM~YS~S. The determination of the concentrations of TI(1). TI(III), and H+ was performed as before? Sulfate was determined as BaSO,(s). NMR Measurements. All NMR measurements were performed on a Bruker AM400 spectrometer at a probe temperature of 0 (i0.5) OC. 20?l NMR spectra were recorded at 230...