Discuss the factors that influence the rate of substitution reactions in octahedral coordination complexes. Illustrate your answer with the substitution of the chloride ligand in $[Co(N{H}_3{)}_{5}Cl{]}^{2+}$ by water, indicating the type of mechanism and why the reaction is relatively slow.

Step-by-step solution

1. State that substitution rates in octahedral complexes are governed by (i) the metal ion’s oxidation state, (ii) the electronic configuration (high‑spin vs low‑spin), (iii) the nature of the ligands (strong‑field or weak‑field), and (iv) the overall geometry.
2. Explain that a higher positive charge on the metal increases the metal‑ligand bond strength, slowing substitution.
3. Mention that strong‑field ligands (e.g., NH₃, CN⁻) create large Δ₀, leading to low‑spin configurations with paired electrons, which reduce the availability of vacant orbitals for incoming ligands, thus decreasing the rate.
4. Describe the two main mechanisms: associative (A) where the entering ligand first forms a 7‑coordinate intermediate, and dissociative (D) where a ligand leaves before the new one attaches, forming a 5‑coordinate intermediate.
5. For $[Co(N{H}_3{)}_{5}Cl{]}^{2+}$, identify the metal as Co(III) (d⁶ low‑spin). Co(III) complexes are known to be substitution‑inert because of the large crystal‑field stabilization energy.
6. State that the strong NH₃ ligands create a stable octahedral field, and the chloride ligand is bound strongly due to the high charge on Co(III).
7. Conclude that the substitution proceeds via a dissociative mechanism: first the chloride ion departs, forming a five‑coordinate intermediate $[Co(N{H}_3{)}_5{]}^{3+}$, followed by coordination of water.
8. Emphasise that the rate is slow because the formation of the five‑coordinate intermediate is energetically unfavourable for a low‑spin d⁶ ion, and the high activation energy reflects the inertness of Co(III).
9. Summarise the answer in one concise statement (as required for the final answer field).

Concept tested: Kinetics of substitution in coordination compounds

Published 4 September 2026 · How this solution was prepared and checked