Most kinetic studies on substitution reactions have been performed in aqueous media. Consider the following reaction where ligand "X" must be replaced by ligand "Y" in the coordination sphere. Since water is abundant, replacement of 'X' with water will be done first followed by inclusion of 'Y'.
The reaction in the aqueous medium in which a water molecule replaces a ligand coordinated by the complex species is called Aquation Reaction or Acid Hydrolysis Reaction. Mechanism of acid hydrolysis when no inert ligand in the complex is a pi donor or pi acceptor. Since in both the SN1 and SN2 pathways, the rate of oxidation depends on the concentration of the [MA5X]n+ complex, it would be difficult to determine whether oxidation would occur via the SN1 and SN2 pathways.
It was found that if the positive charge on the reacting complex ion increases, its conduction velocity decreases. The conduction velocity of the first reaction is about 100 times faster than the second reaction. Based on the above, the aquation process going through the SN1 dissociative mechanism would be a very slow process because as the charge on the substrate increases, the dissociation of the leaving group 'M' from the CMI would become larger.
If the reaction took place via the SN2 mechanism, the increase in the charge on the substrate would be important, because with the increase in the charge on the ion, the input ligand H2O would be strongly attracted to the reacting complex, which would stimulate the feasibility of the formation of coordinate 7. thus, the SN2 mechanism.
Inductive Effect of Inert Ligand
Solvation Effect
Solvation Effect contd…
The reacting species, the intermediate state and the final product are all in the form of hydrated species
Hydration of any species decreases its energy and thus causes its stabilization. Therefore greater the
Greater the charge and smaller the size of the
The five-coordinate intermediate state formed by the SN1 dissociation mechanism would be smaller in size compared to seven. Since the five-coordinate intermediate state is smaller in size, it would undergo a greater amount of hydration, thus becoming more stable than the seven-coordinate intermediate. Therefore, the oxidation of octahedral complexes would prefer to follow the SN1 dissociative mechanism rather than the SN2 associative mechanism.
Acid Hydrolysis
The difference in the rate of aquacation can be explained by considering the pi-bonding capacity of the OH ligand. The coordinated OH ligand has filled p-orbitals capable of pi-bonding with the empty orbitals of the central metal ion. However, in the case of NH3, no such orbitals are present, the only lone pair of electrons being already used to attach to the central metal ion through the formation of coordinate bond.
Due to the stabilization of the SP intermediate, the aquatation of the [Co(en)2(OH)Cl]2+ complex occurs much more easily than the aqualation of the [Co(en)2(NH3)Cl]2+ complex, whose Intermediate SP cannot be stabilized due to the absence formation of pi bonds between the coordinated ligand (NH3) and the central metal ion. The rate constants show that the aqueous solution of [Co(en)2(OH)Cl]+ is much faster than the aqueous solution of [Co(en)2(NH3)Cl]+. It has been observed that the aquaation of such trans complexes proceeds with the formation of a trigonal bipyramidal (TBP) intermediate, which can be stabilized by the formation of a pi bond. The TBP intermediate is stabilized by a pi bond formed by overlapping the empty d orbital of the central metal ion with the filled p orbital of the coordinated OH-ligand in the intermediate TBP species. This process proceeds through the SN1 mechanism, which involves the formation of a TBP intermediate. 1 equitorial bond breaks and changes the square basis to a triangular basis).
Thus in the relationship of trans [Co(en)2(OH)Cl]+ a more stable TBP intermediate is formed rather than a less stable SP intermediate. NO2 is the inert ligand, Cl- is the leaving group and Co is the CMI. One of the filled orbitals of Co CMI overlaps with the vacant p orbital of NO2 group (inert ligand) forming a pi bond.
The lone electron pair of the incoming H2O ligand will experience less repulsion of the electrons from Co from the direction cis to the leaving group or trans to the inert group. Therefore, the presence of pi-acceptor ligands such as NO2 in the complex will facilitate the formation of the Co-H2O bond and thus an associative SN2 mechanism for. The degree of pi overlap when the inert NO2 ligand is cis to the leaving group is much less compared to when the inert NO2 ligand is trans to the leaving group (there will be greater repulsion between electrons from Co and incoming ligand ).
This would result in less electron withdrawal from Co CMI, so Co – H2O bond formation would not be as easy. As a result, the reaction of the complex in which the inert pi acceptor ligand is trans to the leaving group is faster and easier than when the inert pi acceptor ligand is cis to the leaving group. Consequently, the reaction of cis [O2NCo(en)2Cl]+ is slower than the reaction of trans [O2NCo(en)2Cl]+.
Mechanism of acid hydrolysis when the inert ligand is a pi acceptor Intermediates formed during aqualation of complexes containing inert pi. If the H2O ligand attacks the Co CMI from a position cis to the leaving group and passes to the inert ligand, this will lead to the formation of a pentagonal.
