Nucleophilic square-planar substitutions. II. Substitutions of the bromodiethylenetriamineplatinum(II) complex by pyridine derivatives

Abstract

The second-order rate constants for the replacement of coordinated bromide from bromodiethylenetriamineplatinum(11) bromide by a series of pyridine derivatives have been determined over a range of temperatures and the Arrhenius parameters calculated. With the exception of the cyano derivative, replacements of 2-substituted pyridines are considerably slower than those by the corresponding 3- or 4-substituted nucleophiles. This is explicable on the basis of the associative mechanism by invoking steric effects of the substituents. The activation energies for the reaction depend mainly on a combination of two factors, steric and inductive, with the former predominating. These decrease as the position of a given substituent in the nucleophile changes in the order 2, 3, and 4. The inductive effects of the various substituents may be demonstrated with the 4-substituted pyridines, which are sterically similar to the unsubstituted pyridine itself. The methyl substitutent has a small effect, while the cyano and chloro substituents lead to significant increases in activation energy.