Experimental and calculated complex formation curves for a labile metal-ligand system a differential pulse polarographic study of the Pb(II)-(N,N,N′,N′-tetramethylethylenediamine-OH system at fixed ligand to metal ratio and varied pH

Abstract

Experimental and calculated complex formation curves (ECFC and CCFC) for a labile metal-ligand system are defined and used for the speciation study by differential pulse polarography (DPP) at fixed total ligand (LT) to total metal (MT) concentration ratio and varied pH. The CCFC is used for the modeling of species formed in a solution and optimization of formation constants for these species. The CCFC can be generated for any metal-ligand model, including polynuclear metal species, for any LT:MT ratio, and for more than one ligand competing in the complex formation reaction. The metal-ligand model and formation constants are optimized by solving mass-balance equations written for the assumed model and by fitting the CCFC into the ECFC. The methodology of building up the metal-ligand model with initial values for the formation constants, used for further refinement, is described. Lead(II) complexes with the ligand N,N,N′,N′-tetramethylethylenediamine (TMen) are studied by DPP, at 298 K and μ = 0.5 moll−1, at large LT: MT ratios and varied pH. Three lead complexes with the ligand TMen, viz. PbL2+, PbL22+ and PbL2(OH)+, were found and their overall stability constants, as log β, were estimated to be 2.56 ± 0.03, 4.86 ± 0.03 and 9.93 ± 0.02 respectively (all known hydroxide species of lead, including polynuclear species, were incorporated into the PbTMenOH system). The results obtained for PbTMen seem to be reasonable when compared with those for Pb-ethylenediamine reported previously [I. Cukrowski et al., Polyhedron 14 (1995) 1661].