EQUILIBRIA AND DYNAMICS OF TL(EDTA)X2- COMPLEXES (X = HALIDE, PSEUDOHALIDE) STUDIED BY MULTINUCLEAR NMR

Equilibria and dynamics in aqueous solutions of mixed complexes Tl(edta)X2- containing ionic medium (1 M NaClO4) have been studied by means of Tl-205-, N-15-, C-13-, and H-1-NMR and potentiometry. Individual Tl-205 chemical shifts have been determined for all the investigated complexes (X = H2O, OH, Cl, Br, CN, SCN). For the pseudohalide complexes the N-15- and C-13-NMR shifts as well as the spin-spin coupling constants J(Tl-205-N-15) and J(Tl-205-C-13) have also been determined. The coupling constants1 J(Tl-205-C-13) for the complex Tl(edta)CN2- is remarkably large, 10 479 Hz, indicating a very strong thallium-carbon bond. The stability constants, K(X) = [TI(edta)X2-]/{[Tl(edta)-][X-]}, determined by a combination of Tl-205- and C-13-NMR and potentiometry, for X = Cl, CN, and SCN are, respectively: log K(X) = 2.6 (+/-0.1 = sigma), 8.72 (+/-0.03) and 2.70 (+/-0.03). The kinetics for the X-ligand exchange was found to follow the rate equation -d[Tl(edta)X2-]/dt = k(d)[Tl(edta)X2-] + k2[Tl(edta)X2-][X-] which can be ascribed to the reactions [GRAPHICS] where k(d) is greater-than-or-equal-to 1.7 x 10(4), 2.7 (+/-0.1 = sigma) X 10(4), <3, and 2.4 (+/-0.1) x 10(4)s-1 for X = Cl, Br,CN, and SCN, respectively, and k2 is 1.5 (+/-0.1) X 10(6) and 3.0 (+/-0.2) x 10(6) M-1 s-1, for X = CN and SCN, respectively. In order to facilitate the discussion of reaction mechanisms, the crystal and molecular structure of the compound Na2[Tl(edta)CN].3H2O has been determined using single-crystal X-ray diffraction. This compound crystallizes in the monoclinic space group P2(1)/c (no. 14) with a = 7.769 (2) angstrom, b = 14.130 (6) angstrom, c = 17.069 (5) angstrom, beta = 101.44 (2)-degrees, and Z = 4. Thallium is coordinated by edta (hexadentate coordination) and cyanide; i.e., no water molecule is present in the first coordination sphere of thallium. The thallium atom is about 0.08 angstrom below the distorted plane of the four coordinated edta oxygens. Taking into account that the values of the formation rate constants k(f)X for at least X = Br and SCN are close to each other, a mechanism involving dissociation of a water molecule from the Tl(edta)aq- complex as a rate-determining step can be proposed for reaction type a. The rate constant for this step can be estimated to k(w) approximately 1.3 X 10(8) s-1 (for dissociation of one particular water molecule). A similar rate-determining step can be proposed for reaction b. In this case k(w) approximately 1.9 . 10(8)s-1, close to the one for reaction a and slightly higher than the corresponding value (5 x 10(7) s-1) for the aquated Tl3+ ion, in agreement with the expected influence of the decrease of the coordination number of thallium(III) from Tl(edta)aq- to Tl(aq)3+.