The affinity of bismuth(III) for nitrogen-donor ligands

Abstract

Differential pulse polarography has been used to determine the formation constants of Bi³⁺ at ionic strength 0.5 mol dm^–3 and 25 °C with the ligands glycine, trien (triethylenetetramine), bipy (2,2′-bipyridine), [15]aneN₄(1,4,8,12-tetraazacyclopentadecane) and triethanolamine. The equilibria between Bi³⁺ and these ligands are established slowly on the polarographic time-scale, so that separate peaks occur in the differential pulse polarograms for free metal ion and complex, greatly simplifying calculation of the formation constants. Stability constant values (log K₁) for the complexes of Bi^III with these ligands were 10.0 (glycinate), 21.9 (trien), 4.5 (bipy) and 9.2 (triethanolamine). For the macrocycle [15]aneN₄, log K₁(M³⁺+ L ⇌ ML³⁺)= 23.5; for the equilibria ML³⁺+ OH^–⇌ ML(OH)²⁺ and ML³⁺+ H⁺⇌ M(HL)⁴⁺, values of log K were 8.5 and 3.5 respectively. These results are shown to be reasonable in terms of the formation constant log K₁(NH₃)= 5.0 for Bi³⁺ predicted by a dual basicity equation developed previously.