Palladium(II) Halide Complexes III. Acid Hydrolyses and Halide Anations of cis- and trans-Dichlorodiaquapalladium(II) and -Dibromoaquapalladium(II))

Chloride and bromide anations of PdX(H2O)3+, cis-PdX2(H2O)2 and trans-PdX2(H2O)2, acid hydrolyses of PdX3H2O−, cis-PdX2(H2O)2 and trans-PcX2(H2O)2, X = Cl, Br, have been studied at different temperatures by means of a stopped-flow technique. Rate constants and activation parameters are given. The palladium complexes react about 5×104 to 5×105 times faster than the analogous platinum(II) complexes. The activation enthalpies are 8 to 10 kcal mol−1 smaller for palladium. A simple model for description of the rates of acid hydrolyses and halide anations of palladium(II) and platinum(II) halide complexes by cis-, trans-, and ionic charge effects is introduced. For palladium, the relative trans-effect Cl/H2O is about 50 and Br/H2O about 500. The relative cis-effect H2O/ Cl is about 4 and H2O/Br about 8. The trans-effects are 6 times smaller than for corresponding platinum complexes, the cis-effects somewhat greater.
The equilibrium between cis-PdX2(H2O)2 and trans-PdX2(H2O)2 has been studied. The cis-isomer is the predominant species at equilibrium, the equilibrium constant cis/trans being 2.1±0.3 for X = Cl and 6.0± 0.8 for X = Br at 25°C. The ionic strength was 1.00 M and the supporting electrolyte perchloric acid.