Questions: Reaction Mechanisms of Coordination Compounds (Substitution)

Question 1

Most octahedral substitution reactions proceed by a dissociative interchange (Id) mechanism rather than a purely associative (A) mechanism. What is the primary reason?

• A. Octahedral complexes are always more thermodynamically stable, preventing association
• B. The metal center in an octahedral complex is sterically crowded by six ligands, making it difficult for a seventh ligand to approach — bond breaking (dissociation) must precede or accompany bond making
• C. The CFSE of the seven-coordinate transition state is always zero
• D. Octahedral complexes cannot change their coordination number under any circumstances

Answer: (B)

Steric crowding at the metal center is the dominant factor. Six ligands around a metal ion leave little room for a seventh to approach, making a purely associative mechanism (increasing coordination number to 7 in the transition state) energetically costly. Instead, the departing ligand must partially dissociate (lengthening its bond) before or as the incoming ligand begins to bond. This is the dissociative interchange (Id) pathway. Square planar complexes, by contrast, have open coordination sites above and below the plane, readily accommodating a fifth ligand — which is why they favor associative (A) mechanisms.

Question 2

The trans effect in square planar Pt(II) chemistry refers to the ability of certain ligands to labilize the ligand trans to themselves, facilitating its substitution.

• T. True
• F. False

Answer: True

The trans effect is a kinetic phenomenon: ligands with a strong trans effect (like CO, CN⁻, C₂H₄, PR₃) weaken the bond to the ligand directly opposite them in a square planar complex, making that position more susceptible to substitution. The trans influence is the related ground-state thermodynamic effect (weakening of the trans M-L bond length). The trans effect order roughly follows: H₂O < NH₃ < Cl⁻ < Br⁻ < I⁻ < NO₂⁻ < CO ≈ CN⁻ ≈ C₂H₄. This effect is critically important in synthesis — it allows chemists to direct which ligand is replaced, enabling stereoselective preparation of specific isomers like cisplatin.

Question 3

d³ and low-spin d⁶ octahedral complexes are kinetically inert, while d⁰ and high-spin d⁵ complexes are kinetically labile.

• T. True
• F. False

Answer: True

Kinetic inertness correlates with crystal field activation energy (CFAE) — the loss of CFSE when moving from the octahedral ground state to the transition state geometry. d³ (t₂g³) and low-spin d⁶ (t₂g⁶) configurations have large CFSE in the octahedral geometry and lose significant stabilization in any transition state with a different coordination number, creating a large activation barrier. d⁰ has no d-electrons and thus no CFSE to lose, while high-spin d⁵ (t₂g³ eg²) has zero CFSE (each orbital singly occupied), so neither configuration faces a CFSE-based barrier. This is why Cr³⁺ (d³) and Co³⁺ low-spin (d⁶) complexes can be isolated as specific isomers that persist for days, while [Mn(H₂O)₆]²⁺ (d⁵ high-spin) exchanges water molecules in microseconds.

Question 4

Explain how the trans effect is used to synthesize cisplatin, [Pt(NH₃)₂Cl₂], selectively rather than transplatin.

Model answer: Starting from [PtCl₄]²⁻, the first substitution replaces one Cl⁻ with NH₃ to give [PtCl₃(NH₃)]⁻. For the second substitution, the trans effect determines which Cl⁻ is replaced. Cl⁻ has a stronger trans effect than NH₃, so it labilizes the ligand trans to itself more than NH₃ does. The Cl⁻ trans to the newly added NH₃ is less labile (NH₃ has weak trans effect), while the Cl⁻ trans to another Cl⁻ is more labile (Cl⁻ has moderate trans effect). The incoming second NH₃ therefore replaces a Cl⁻ that is trans to Cl⁻, placing the second NH₃ cis to the first. This gives cisplatin. To make transplatin, you start from [Pt(NH₃)₄]²⁺ and add Cl⁻, which replaces NH₃ trans to NH₃ — but since all positions are equivalent initially, selective trans synthesis from the tetrachloride is the standard route to the cis isomer.

This synthesis elegantly demonstrates how understanding mechanism enables stereocontrol. Cisplatin is one of the most important anticancer drugs, while transplatin is therapeutically inactive — so the ability to selectively make one isomer is medically critical.