The trans-effect in octahedral complexes. Part II. Anation reactions of some trans-Rh en2L(OH2)2+complexes

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Abstract
The kinetics of anation reactions of trans-Rh en2L(OH2)2+ with X–(L and X = Cl, Br, or I) have been studied and rate parameters obtained. Combination of these parameters with thermodynamic and kinetic parameters obtained previously enables the relative stabilities of the dihalogeno-complexes and their corresponding aquohalogeno-complexes to be calculated. Values for the relative enthalpies of the aquochloro-, aquobromo-, and aquoiodo-complexes in aqueous solution have also been calculated, and the results show that H2O is a “harder” ligand than chloride. The kinetic trans-effect of L on the Rh–OH2, bond is I > Br > Cl. Comparison with data for the trans-effect on rhodium–halide bonds shows that the magnitude of the trans-effect increases with increasing “hardness” of the leaving group. The relative nucleophilic character of the halides towards the aquo-complexes shows a small but definite trend in the sense I > Br > Cl, when measured by activation enthalpies. Although the intimate mechanism of the reactions is not deducible from the kinetic data, the mode of operation of the thermodynamic and kinetic trans-effects is explicable in terms of differential trans-bond weakening brought about by different degrees of covalency in the bond between the trans-labilising ligand, L, and the metal. The results correlate well with Pearson's “hard” and “soft” terminology.

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