Hydrogen Carbonate Inhibited and Induced Substitution Reactions of Labile Square-Planar Diethylenetriaminepalladium(II) Complexes

The reaction of labile square-planar diethylenetriaminepalladium(II) complexes with hydrogen carbonate ion was studied by stopped flow spectrophotometry as a function of [HCO⁻₃] and [Cl⁻] and constant [Pd], ionic strength and temperature to investigate the substitution behaviour with a nucleophile capable of affecting the solution pH. The kinetic results significantly deviate from those for normal square-planar substitution behaviour. The reaction comprises two consecutive equilibrations, which have complex kinetics compared to the simple two-term rate law normally encountered, and which represent hydrogen carbonate inhibited and induced substitution reactions, respectively. The suggested mechanism and data treatment account for the observed deviating kinetics and provide alternative ways of obtaining formerly determined aquation and anation rate constants with comparable accuracy, even for complexes too labile to be monitored directly.