Formation constants of lanthanide (III)- aminopolycarboxylate - ATP mixed ligand complexes and their systematics

Abstract

Formation constants (log <em>K</em> , log <em>K</em> and log <em>K</em>) of mixed ligand lanthanide(III) complexes of the type [Ln(III).A.ATP)²,where Ln(III) = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺ or Dy³⁺; A=NTA (nitrilotriacetate) or HEDTA (2-hydroxyethylethylenedia-mine triacetate) and ATP = adenosine 5'-triphosphate (L), and of the binary [Ln(III).ATP] and [Ln(III).(ATP)₂] complexes have been determined by potentiometric <em>p</em>H titrations using the Irving-Rossotti approach at three temperatures 20, 30 and 40°C and at a fixed ionic strength, <em>I</em> = 0.2 mol drn^-3 (NaC1O₄). The solution stabilities (log <em>K</em> values) are influenced by the electrostatic effect involved in ternary complexation and increase with temperature. The enthalpy factor (D<em>H</em>) has been found to be small but unfavourable and the entropy factor (D<em>S</em>) large and favourable. The log <em>K</em> values lie in the order NTA > HEDTA with respect to A and La³⁺ < Ce³⁺ < Pr³⁺ < Nd³⁺ < Sm³⁺ < Eu³⁺ > Gd³⁺ < Tb³⁺ < Dy³⁺ with respect to lanthanides. Tetrad effect is present in the formation constant values; its magnitude has been found to lie in the sequence <em>f</em>¹> <em>f</em>³<em>- f</em>⁴<em>≈</em> <em>f¹⁰- f¹¹</em> for the Ln(III) ions. Systematics in the formation constant values has been further studied by evaluating changes in the inter-electronic repulsion Racah parameters, extrastabilisation of specific 4<em>f</em>⁹-configurations and nephelauxetic ratio using experimental values of the formation constants.