Dynamics of Ligand Substitution in Metal Complexes : A Survey of Studies on Octahedral and Square-Planar Complexes in Solution

Department of Chemistry, Inorganic Chemistry Division,

University College of Science, Calcutta 700009.

In connection with any chemical reaction two kinds of information are of vital importance viz. (i) how far a reaction would proceed and (ii) how fast will it reach the goal i e. the dynamics of the process. The former is governed by thermodynamic and the latter by kinetic considerations. However, the recognition of the importance of kinctic aspects of inorganic reactions is a fairly recent one. Some forty years ago inorgan c chemists used to correlate the failure of a substance to undergo a change with inherent stability of the substance. This was based on the then prevailing misconception that unlike the 'slow' organic reactions the inorganic reactions are 'instantaneous'. While this Is true for the vast majority of simple inorganic reactions, e. g. reactions between ions forming an insoluble product thus leading to the formation of a precipitate, combination between H⁺ and OH^-tons forming water (for which the rate constant as measured¹ by the pressure-jump relaxation technique is now known to be 1.5 x 10¹¹ M^-1 sec^-1 at 25°C), and so on, it was later recognised that many of the inorganic reactions are also very slow Thus, aquation of Ir(NH₃)₅ I²+ involving replacement of the bound Iodide by H₂O in acid solution has a rate constant of 1 9 x 10^-10 sec^-1 at 25°C². Our present state of knowledge on the mechanistic aspects of ligand substitution in octahedral and square-planar complexes is presented below.