Thallium(III) coordination compounds: chemical information from 205 Tl NMR longitudinal relaxation times

205 Tl longitudinal relaxation rate measurements were performed on several thallium(III) complexes with the composition Tl(OH) n (H 2 O) 6− n (3− n )+ ( n = 1,2), Tl(Cl) n (H 2 O) m − n (3− n )+ , Tl(Br) n (H 2 O) m − n (3− n )+ ( m = 6 for n = 1–2, m = 5 for n = 3, m = 4 for n = 4), Tl(CN) n (H 2 O) m − n (3− n )+ ( m = 6 for n = 1–2, m = 4 for n = 3–4) in aqueous solution, at different magnetic fields and temperatures. 13 C and 2 D isotopic labelling and 1 H decoupling experiments showed that the contribution of the dipolar relaxation path is negligible. The less symmetric lower complexes ( n < 4) had faster relaxation rate dominantly via chemical shift anisotropy contribution which depended on the applied magnetic field: T 1 values are between 20 and 100 ms at 9.4 T and the shift anisotropy is Δσ = 1000–2000 ppm. The tetrahedral complexes, n = 4, relax slower; their T 1 is longer than 1 s and the spin–rotation mechanism is probably the dominant relaxation path as showed by a temperature dependence study. In the case of the TlCl 4 − complex, presumably a trace amount of TlCl 5 2− causes a large CSA contribution, 300 ppm. Since the geometry and the bond length for the complexes in solution are known from EXAFS data, it was possible to establish a correlation between the CSA parameter and the symmetry of the complexes. The relaxation behaviour of the Tl–bromo complexes is not in accordance with any known relaxation mechanism. Copyright © 2002 John Wiley & Sons, Ltd.