SCN^- effect on the palladium(II) transfer in two and three phases systems using triphenylphosphine sulfide as a carrier

The extraction of Pd(II) by triphenylphosphine sulfide (TPPS) in toluene from hydrochloric acid solutions containing small amounts of SCN- has been investigated. The role of thiocyanate on the Pd(II) extraction is explained as a trans-effect with the formation of mixed complexes PdCl4-n(SCN)2-n, less inert to ligand substitution than the chloride complexes. The analysis of metal distribution data by numerical calculations suggested the formation of the species Pd(SCN)2·TPPS and Pd(SCN)2·2TPPS in the organic phase with formation constants log K021 = 13.00 and log K022 = 14.24, respectively. A comparison of the results with those obtained using triisobutylphosphine sulfide (TIBPS) under similar conditions reveals a lower influence of SCN- concentration and also lower values for Pd(II) extraction, which can be attributed to the electron withdrawing characteristics of the aromatic ring with a decrease of the solvating power of the S atom for TPPS. The results obtained on Pd(II) distribution have been exploited in a three phases system by implementing a solid supported liquid membrane, being thiocyanate concentration the driving force of the mass transfer process. The behavior of the system is similar to that observed in the two phases.