Equilibrium and structure of thallium(III)–ethylenediamine complexes in pyridine solution and in solid

The formation of three [Tl(en)n]3+ complexes (n=1–3) in a pyridine solvent has been established by means of 205Tl and 1H NMR. Their stepwise stability constants based on concentrations, Kn=[Tl(en)n 3+]/{[Tl(en)n−1 3+]·[en]}, at 298 K in 0.5 M NaClO4 ionic medium in pyridine, were calculated from 205Tl NMR integrals: log K1=7.6±0.7; log K2=5.2±0.5 and log K3=2.64±0.05. Linear correlation between both the 205Tl NMR shifts and spin–spin coupling 205Tl–1H versus the stability constants has been found and discussed. A single crystal with the composition [Tl(en)3](ClO4)3 was synthesized and its structure determined by X-ray diffraction. The Tl3+ ion is coordinated by three ethylenediamine ligands via six N-donor atoms in a distorted octahedral fashion.