MIXED-LIGAND CHELATE COMPLEXES OF THALLIUM(III), CHARACTERIZED BY EQUILIBRIUM MEASUREMENTS, NMR AND RAMAN-SPECTROSCOPY, AND X-RAY CRYSTALLOGRAPHY

Equilibria in aqueous solutions at ionic strength I = 1.00 (NaClO4 or NaNO3) and 25 degrees C between the 1:1 thallium(III) complexes [T1(L) ] (L are diaminotetraacetate ligands edta and cdta and the hexaamine t pen {tetrakis(2-pyridylmethyl)ethylenediamine}) and different coligand s A (A are OH-, Cl-, Br-, I-, SCN-, N-3(-), py, en, Hen(+), phen, bpy, gly(-), Hgly, ox(2-), and ida(2-)) have been investigated by means of potentiometric pH titrations and C-13 and H-1 NMR, Raman, and IR spec troscopy. Each ligand gives rise to the following equilibria, allowing the determination of the corresponding equilibrium constants by pH me asurements: (1) [Tl(L)(H2O)](n) reversible arrow[Tl(L)(OH)](n-1) + H+; (2) [Tl(L)(H2O)](n) + A reversible arrow [Tl(L)(A)](n) + H2O. The pre sence of [Tl(L)(A)] in solution and the corresponding data have been c onfirmed by NMR for [Tl(edta)(A)] and [Tl(cdta)(A)]: Although the C-13 coordination chemical shifts are small, heteronuclear coupling consta nts J(Tl-205/203, C-13) and, in particular, those involving the carbox ylate function can be employed for monitoring structural changes and t he determination of equilibrium constants. The cdta complexes, having a rigid stereochemistry imposed by the cyclohexyl ring, in solution ex hibit conformations for the glycine-type chelate rings which are close to those found in the solid for analoguous edta complexes. The parame ter (3)J(T1,H) for the acetate groups of the cdta ligand established c onformational similarities for all the complexes with monodentate coli gands A, different from those coordinating in a bidentate fashion, sug gesting that also the complex [Tl(L)(H2O)(n)](-) exists as a monoaquo species, i.e., n = 1. Two-dimensional NMR for [Tl(cdta)(phenH)] and [T l(cdta)(phen)](-) provides simple and direct evidence for the formatio n of stable mixed-ligand complexes with CN = 7 and 8 in aqueous soluti on. Raman spectra of solutions with equilibria of types (1) and (2) fo r L = edta allow the assignment of bands to the mixed-ligand complexes present in agreement with the equilibrium constants obtained. The X-r ay crystal structures of two complexes which were isolated in crystall ine form, i.e., Ca[Tl(edta)(OH)]. 3H(2)O (monoclinic, space group P2(1 )/c, a = 13.744(8) Angstrom, b = 9.141(2) Angstrom, c = 13.752(0) Angs trom, beta = 100.71(0)degrees, Z = 4) and [Tl(tpen)(NO3)](ClO4)(2) (mo noclinic, space group P2(1)/c, a = 12.718(6) Angstrom, b = 13.640(7) A ngstrom, c = 18.883(9) Angstrom, beta = 108.84(4)degrees, Z = 4) showe d a CNs of 7 for the former and 8 for the latter Tl(III) complex salt.