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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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