The effects of liganddecomposition on the pseudo first-order profile of a ligand substitution reaction: a “silent killer” in the background

Abstract

The effect of ligand decomposition on the apparent rate constant for a ligand substitution reaction where the reverse reaction is negligible is examined. When the ligand concentration remains in excess for the entirety of the reaction, the data can be fitted to a modified form of the familiar pseudo first-order rate expression. However rapid ligand decomposition may result in the ligand concentration not remaining in excess for the duration of the reaction. Data simulations show that in this latter case the data still fit remarkably well to a first-order rate equation. Plots of the apparent rate constant versus the initial ligand concentration are also linear. However the reaction will not proceed to completion despite the reverse reaction being insignificant. Furthermore intercepts in the plots of apparent rate constant versus the initial ligand concentration are obtained, in addition to misleading values of the second-order rate constants for the ligand substitution reaction. We show that kinetic coupling between the decomposition of the ligand and its complex formation reaction may easily lead to falsified conclusions. Thus, the first-order appearance of a kinetic trace does not guarantee real first-order behavior.