CSIR-NET Inorganic Reaction Mechanisms — Substitution and Redox
Coordination chemistry questions in CSIR-NET Part C increasingly ask why a reaction gives one product and not another, not just what the product is. Two mechanistic ideas answer almost every such question: the trans effect governing ligand substitution at square-planar centres, and the inner-sphere/outer-sphere distinction governing how electrons actually move in redox reactions between metal complexes.
Square-planar substitution and the trans effect
The trans effect is the ability of a ligand already bound to a metal to labilise (make more reactive) the ligand positioned trans to itself, accelerating substitution at that position. It is a kinetic phenomenon, distinct from the trans influence (a ground-state, thermodynamic effect on bond length). The commonly quoted order, strongest to weakest trans-director:
Worked example 1 — synthesising cis- and trans-platin
Q. Explain, using the trans effect, why treating [PtCl₄]²⁻ with NH₃ gives cis-[Pt(NH₃)₂Cl₂], while treating [Pt(NH₃)₄]²⁺ with Cl⁻ gives the trans isomer.
Route to cis-platin. Label the four square-planar positions 1–4, with 1 trans to 3 and 2 trans to 4. Starting from [PtCl₄]²⁻ (Cl at all four positions), the first NH₃ substitutes any Cl — say position 1. Now position 3 is trans to NH₃ (a weak trans-director, so the Cl there is not very labile), while positions 2 and 4 are trans to each other — each is strongly labilised by the Cl opposite it. The second NH₃ therefore substitutes at position 2 (or equivalently 4), giving NH₃ at 1 and 2 — which are cis to each other — with the remaining Cl at 3 and 4.
Route to trans-platin. Starting from [Pt(NH₃)₄]²⁺, the first Cl⁻ substitutes any NH₃ — say position 1. Now position 3, trans to this strongly-labilising Cl, is the most reactive site. The second Cl⁻ substitutes there, putting Cl at positions 1 and 3 — which are trans to each other.
The two syntheses differ only in which ligand is introduced first — the strong trans-director (Cl) always directs the next substitution to the position opposite itself, and tracking that single rule through two steps predicts the geometry correctly every time.
Associative vs dissociative pathways
Square-planar Pt(II) substitutions are typically associative (A), proceeding through a five-coordinate trigonal-bipyramidal intermediate, and usually follow a two-term rate law:
where k₁ is the solvent-assisted (pseudo-first-order) pathway and k₂ is direct attack by the incoming nucleophile Y. A negative activation volume (ΔV‡ < 0) is the classic kinetic signature of an associative mechanism — the transition state is more crowded, hence more compact, than the starting complex.
Worked example 2 — reading a rate law for mechanism type
Q. A substitution reaction on a square-planar Pt(II) complex shows rate dependence on the concentration of the incoming ligand Y, and the reaction accelerates under high pressure. What does this indicate about the mechanism?
Dependence on [Y] indicates a k₂ (associative, bimolecular) contribution to the rate — Y is directly involved in the rate-determining step. Acceleration under pressure corresponds to a negative activation volume, meaning the transition state occupies less volume than the reactants — consistent with formation of a crowded five-coordinate intermediate. Both observations together confirm an associative (A) pathway, not a dissociative one.
Inner-sphere vs outer-sphere electron transfer
Redox reactions between metal complexes proceed by one of two mechanisms. In an outer-sphere mechanism, the electron tunnels between two intact coordination spheres that never share a ligand — no bonds break or form at either metal. In an inner-sphere mechanism, a ligand bridges the two metal centres momentarily, providing a pathway for the electron and typically ending up transferred to the other metal in the process.
Worked example 3 — Taube's bridging-ligand experiment
Q. [Co(NH₃)₅Cl]²⁺ (inert, substitution-resistant Co(III)) reacts with [Cr(H₂O)₆]²⁺ (labile Cr(II)). After the redox reaction, the chromium product is isolated as [CrCl(H₂O)₅]²⁺. What does this tell you about the mechanism?
Cr(III) (the product oxidation state) is substitution-inert, just like the Co(III) starting complex — so the Cl⁻ that ends up bound to chromium could only have arrived there while Cr was still labile Cr(II), i.e. during the electron-transfer step itself. This is direct evidence that the Cl⁻ ligand bridged both metal centres and was transferred along with the electron — the signature of an inner-sphere mechanism. This is essentially Taube's classic demonstration that established bridged-ligand electron transfer as a real pathway, distinct from simple outer-sphere tunnelling.
Common mistakes that cost marks
- Confusing associative (A) / dissociative (D) with SN1 / SN2 organic nomenclature. They describe analogous kinetic ideas but are formally separate vocabularies — inorganic mechanism questions expect A/D/Ia/Id, not SN1/SN2.
- Mixing up trans effect and trans influence. Trans effect is kinetic (rate of substitution); trans influence is thermodynamic (ground-state bond weakening/ lengthening). A ligand can rank differently on the two scales.
- Assuming every electron-transfer reaction is outer-sphere by default. Inner-sphere transfer requires a ligand capable of bridging (a halide, cyanide, or pseudo-halide with a lone pair to spare); outer-sphere is the default only when no such bridging ligand is available or substitution is too slow to permit bridging.
- Applying trans-effect reasoning to octahedral complexes without care. The classic trans-effect series above is calibrated for square-planar Pt(II)-type chemistry; octahedral substitution kinetics are governed by a different, less well-generalised set of rules.
Mechanism comparison table
| Mechanism | Characteristic evidence | Example |
|---|---|---|
| Associative (A) | Rate depends on [Y]; ΔV‡ negative | Most square-planar Pt(II) substitutions |
| Dissociative (D) | Rate independent of [Y]; ΔV‡ positive | Most octahedral substitutions with a stable 5-coordinate intermediate |
| Outer-sphere electron transfer | No ligand exchange between reactants; both spheres stay intact | [Fe(CN)₆]⁴⁻ + [Fe(CN)₆]³⁻ self-exchange |
| Inner-sphere electron transfer | Bridging ligand ends up transferred to the other metal | [Co(NH₃)₅Cl]²⁺ + [Cr(H₂O)₆]²⁺ |
Working through a Nernst-equation-based redox potential alongside a mechanism question? The Nernst Equation calculator handles the concentration-dependence calculation so you can focus on the mechanistic reasoning.
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