Determination of the stability constants for cobalt, nickel and palladium homogeneous catalyst complexes containing triphenylphosphine ligands

Homogeneous catalysts are complex compounds that are always in equilibrium with their free metal, free ligand and other forms of complexes. The ratios between different species are defined by the stability constants, which are influenced by different parameters such as the type of metal, ligand, counter ion or solvent. The main goal of this paper is the determination of the stability constants for a range of different homogeneous catalyst complexes and therefore the concentration of each species present in the solution. This information is needed for the modelling and design of reverse flow adsorption (RFA) technology, a novel concept for the recovery and recycling of homogeneous catalysts [J. Dunnewijk, H. Bosch, A.B. de Haan, Sep. Purif. Technol. 40 (3) (2004) 317-320; J. Dunnewijk, H. Bosch, A.B. de Haan, Adsorption 11 (2005) 521-526]. Cobalt, nickel and palladium halogens with triphenylphosphine as a ligand are selected as complexes since they are commonly used in homogeneous catalysis. Titration experiments with UV-vis spectroscopy as analytical technique have been carried out. The results were analyzed with a stability constant model developed for 1:2 complexation and adjusted for easy handling using Microsoft Excel. The stability constants of the selected complex systems increase in order: [PdCl2(OPPh3)2]acetonitrile ˜ [CoCl2(PPh3)2]butanol < [CoBr2(PPh3)2]acetonitrile < [CoCl2(PPh3)2]acetonitrile < [NiBr2(PPh3)2]acetonitrile < [PdCl2(PPh3)2]DMF < [PdCl2(PPh3)2]acetonitrile. The obtained results for the stability constants could be explained with the hard and soft acid base theory in combination with the natural order of different species theory.