Transmission of Electronic Effects in Transition Metal Complexes

Abstract

AN interesting aspect of transition metal chemistry relates to the influence which one ligand partner, in a mixed complex, may exert on another. The importance of electronic effects transmitted through a central metal ion is demonstrated, for example, by the directing influence of certain ligands in substitution reactions1. We report here some preliminary results on the influence of certain anionic ligands on the nature of the metal–pyridine bond in mixed complexes of cobalt (II). Our evidence for transmitted electronic effects in these complexes rests on certain striking differences in the influence of X on the position of equilibrium in the following configurational transformation : in chloroform solution. Py = pyridine and X = Cl−, Br−, I−, OCN−, SCN− or SeCN−. Co(II) complexes are well suited to configurational equilibria investigations of this kind because of the relatively high crystal field stabilization energy expected for the d7 configuration in a tetrahedral field2, and experimentally, because of the separate well-defined visible spectral characteristics of the two stereo -chemical arrangements. Equilibrium constants derived from spectrophotometric measurements were coupled with enthalpies determined calorimetrically and the associated entropy changes calculated. Since chloroform is relatively non-co-ordinating, the derived thermo-dynamic quantities refer, fairly closely, to the actual association reaction as represented in equatior (1).