Ligand effects inner-sphere reactions, kinetics

The protonated aluminol sites are the most effective fluoride sorption sites and are usually responsible for the rapid kinetics due to coulombic attraction between the positively charged sites and the negatively charged fluoride species. The reaction with non-protonated sites involves ligand exchange, leads also to the formation of inner-sphere complexes, releases hydroxyl ions, is slow and characterized by a higher activation energy. 

Therefore, the effect of some ligands on the kinetics of reactions which proceed via the two mechanisms was studied reaction (50) was chosen as an example of a reaction which proceeds via the outer-sphere mechanism 105) and reactions (51)-(53) were chosen as examples of reactions which proceed via the inner-sphere mechanism 25,29,41,61). 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

How can we rationalize the kinetic trans effect One still popular but only partial explanation was devised by A. Grinberg in the 1930s. It maintains that the transdirecting ability of a ligand is related to its polarizability. As noted in the section on inner-sphere electron-transfer reactions, polarizability can be thought of as the ease with which a dipole moment can be induced in a species. Figure 5.13 illustrates... 

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