Dissociative Substitution in Octahedral Complexes

Dissociative substitution in coordination complexes is a reaction mechanism where the rate-determining step involves the bond breaking between the metal center and a ligand, leading to a decrease in the coordination number of the metal before the entering ligand bonds to the metal.

Let's analyze each statement regarding dissociative substitution in an octahedral transition metal complex:

Statement (a): Steric Hindrance and Dissociation Rate

Statement (a) says that high steric hindrance between ligands favors fast dissociation of a ligand. In an octahedral complex, if the ligands are bulky, there is significant steric repulsion between them. This steric strain weakens the metal-ligand bonds. Breaking a strained bond requires less energy. Therefore, increased steric hindrance makes it easier for a ligand to dissociate from the metal center, thus favoring a faster rate for the dissociative step.

This statement is consistent with the principles of dissociative substitution. High steric hindrance facilitates ligand dissociation.

Statement (b): Metal Charge and Ligand Acceptance

Statement (b) says that increased charge on the metal atom/ion favors the acceptance of the electron pair of the entering ligands. A higher positive charge on the metal makes it more electron-deficient and a stronger Lewis acid, which would indeed favor interaction with an electron-donating entering ligand (a Lewis base). However, in a purely dissociative (D) mechanism, the rate-determining step is the dissociation of the leaving group, which is independent of the entering ligand. The acceptance of the entering ligand occurs *after* the intermediate is formed. Furthermore, a higher positive charge generally strengthens the metal-ligand bond that needs to break in the dissociative step, potentially slowing down the reaction rate.

Therefore, statement (b) is not correct for the rate-determining step of a dissociative mechanism.

Statement (c): Pentacoordinated Intermediate

Statement (c) says that a pentacoordinated intermediate is observed. In the dissociative mechanism (D), the leaving group (L) departs from the octahedral complex $[\text{ML}_6]$, forming a five-coordinate intermediate $[\text{ML}_5]$ and the free leaving group L. The entering ligand ($\text{L}'$) then rapidly attacks the five-coordinate intermediate $[\text{ML}_5]$ to form the product $[\text{ML}_5\text{L}']$.

$[\text{ML}_6] \xrightarrow{\text{slow, -L}} [\text{ML}_5] + \text{L}$ (Rate-determining step)

$[\text{ML}_5] + \text{L}' \xrightarrow{\text{fast}} [\text{ML}_5\text{L}']$

The intermediate $[\text{ML}_5]$ is indeed pentacoordinated (coordination number 5).

This statement is correct regarding the dissociative substitution mechanism.

Statement (d): Influence of Entering Ligand

Statement (d) says that the nature of the entering ligand significantly influences the reaction. In a pure dissociative (D) mechanism, the rate-determining step is the dissociation of the leaving group. The entering ligand ($\text{L}'$) is not involved in this slow step. Its concentration and nature only affect the faster second step where it reacts with the intermediate. Therefore, the rate of a purely dissociative reaction is independent of the nature and concentration of the entering ligand.

This statement is not correct for a pure dissociative substitution mechanism.

Conclusion

Based on the analysis of each statement in the context of dissociative substitution in octahedral transition metal complexes:

• Statement (a) is correct.
• Statement (b) is incorrect.
• Statement (c) is correct.
• Statement (d) is incorrect.

The correct statements are (a) and (c).