Determination of the Mechanism of Substitution Reactions of Sterically Hindered Palladium(II) Complexes in Aqueous Solution by the Use of a Volume Profile*

Pressure dependencies of the nucleophile-independent rate constants were studied for the reactions of [Pd(teden)X]n+ ions (where teden = N'-[2-(diethylamino)ethyl]-N,N-diethylethane-1,2-diamine and X = Cl-, Br-, N3-, I-, SCN- and NH3) with various nucleophiles, Y. The partial molar volumes of these complexes were measured at 25�C. By combining these values with the respective volumes of activation, volume profile diagrams were constructed from which the reaction mechanism was determined to be of the interchange type, most likely Ia. For three representative reactions, in which X = Cl- and Br-, and Y = N3- and SCN-, ΔV‡exp values for the nucleophile-dependent path were also derived. The mechanism of these reactions is also considered to be concerted.