Complexation of palladium(II) with monovalent anionic ligands – a spectrophotometric investigation

Abstract

The hydrometallurgical processing of platinum group metals (PGMs) requires exact knowledge of the speciation (identity and distribution of specific metallic species/complexes in solution) of the metal complexes involved. The stability constants (also known as formation constants) convert into Gibbs free energies that, when different, result in a shift of the stability regions of different complexes that are illustrated by a Pourbaix diagram. Information on the stability constants of palladium complexes with monovalent ligands (i.e. thiocyanate, chloride, bromide and mixed palladium(II) chloro-hydroxo and palladium(II) bromo-hydroxo complexes) in solution is not readily available and quite limited. The majority of the methods employed involved the preparation of individual solutions with varying concentrations of the base metal, ligand or both. These techniques yielded only a few useable data points. To improve on this experimental shortcoming, an improved automated titration method was developed and employed to investigate the spectrophotometry of these complexes at 25 C and an ionic strength of 1.0 M. This new experimental method was developed by combining two specialized analytical apparatus that made it possible to automatically vary the ligand concentration over various intervals ensuring a very accurate and consistent set of absorbance data. The ideal titration conditions were determined by employing the Hyperquad Simulation and Speciation (HySS) software, while the stability constants for all of the palladium systems were calculated with the software program HypSpec. HypSpec software is specifically developed for the determination of stability constants from spectrophotometric data. The stability constants, log βn, determined in this study for the palladium(II) thiocyanate complexes [Pd(SCN)n(H2O)4-n]2-n (n = 0 - 4), which are generally accepted to be square planar, are: log β1 = 8.14, log β2 = 15.46, log β3 = 21.94, and log β4 = 27.42. However, a fivecoordinated species, i.e. [Pd(SCN5]3-, with a formation constant of log β5 = 31.94, would seem to exist (proposed to be square pyramidal) and form part of the system. These published results represent the first complete set of communicated stability constants. The experimental procedure was customized to determine the stability constants for the palladium(II)-chloride and -bromide complexes. For [PdCln(H2O)4-n]2-n (n = 0 - 4), the log β values are: log β1 = 4.49, log β2 = 7.80, log β3 = 10.18 and log β4 = 11.54, while the log β values for [PdBrn(H2O)4-n]2-n (n = 0 - 4), are: log β1 = 5.04, log β2 = 9.12, log β3 = 12.38 and log β4 = 14.55. For the chloride system our results represent a refinement of already published data, whereas our results for the bromide system represent only the second full set of published stability constants that are deemed to be much more accurate and reliable. Similarly, the experimental procedure was again modified and applied to the mixed palladium(II) chloro-hydroxo and palladium(II) bromo-hydroxo complexes. The stability constants for the mixed chloro-hydroxo complexes [PdCl4−n(OH)n]2− (n = 0 - 4), are: log β1 = 18.36, log β2 = 23.21, log β3 = 26.91 and log β4 = 29.68. The stepwise stability constants of the palladium(II) bromo-hydroxo system, [PdBr4−n(OH)n]2− (n = 0 - 4), are as follows: log β1 = 18.73, log β2 = 22.25, log β3 = 25.58 and log β4 = 28.47. For the chloro-hydroxo system our results represent a refinement of already published data, whereas our results for the bromohydroxo system represent the first set of stability constants. The newly developed experimental technique, linking and automating a double burette autotitrator with a UV-vis spectrophotometer equipped with a flow-through cuvette, was applied to five different palladium(II) systems with great success. This technique can be applied to similar metal-ligand systems for the accurate determination of stepwise stability constants.