Reaction Mechanisms of Coordination Compounds

Labile vs inert — a KINETIC distinction

• Labile complexes exchange ligands fast; inert complexes exchange ligands slowly.
• These are rate (kinetic) terms — they say how fast a reaction happens, not how far.
• Do not confuse with stable/unstable, which are thermodynamic (equilibrium) terms.
• Classic point: an inert complex need not be thermodynamically stable, and a labile one need not be unstable. Example: [Co(NH₃)₆]³⁺ in acid is thermodynamically unstable (should decompose) yet kinetically inert — it survives for days.

CFSE / d-configuration correlation

• Taube's rule of thumb: complexes with high CFSE in the ground state and no low-energy path to the transition state tend to be inert.
• Inert (octahedral): d³ (e.g. Cr³⁺), and low-spin d⁶ (e.g. Co³⁺, low-spin d⁴, d⁵ also slow).
• Labile: most others — d⁰, d¹, d², high-spin d⁴–d⁷, d⁸ (partial), d⁹, d¹⁰.
• Reason: substitution needs to either lose an electron from a stabilised t_2g or promote one to e_g — costly for d³ and low-spin d⁶.

Octahedral substitution mechanisms

• Dissociative (D): leaving group departs first → 5-coordinate intermediate → new ligand enters. Rate depends mainly on the leaving group.
• Associative (A): incoming ligand adds first → 7-coordinate intermediate → leaving group departs. Rate depends on the incoming ligand.
• Interchange (I): bond-making and bond-breaking are concerted (no true intermediate). Two flavours:
• I_d (dissociative interchange) — bond-breaking dominates; typical for most octahedral substitutions.
• I_a (associative interchange) — bond-making dominates.
• Most octahedral substitutions proceed by D or I_d (dissociative character), since 7-coordination is crowded.

Square-planar substitution & the trans effect

• Square-planar complexes (usually d⁸: Pt²⁺, Pd²⁺, Ni²⁺, Au³⁺) substitute by an associative (A) path via a 5-coordinate trigonal-bipyramidal intermediate.
• Trans effect: the ability of a coordinated ligand to labilise the ligand trans to itself, directing where substitution occurs. (Trans effect = kinetic; the related ground-state weakening is the trans influence, a thermodynamic idea.)

Application — making cis- vs trans-[Pt(NH₃)₂Cl₂]:

• cis (cisplatin): start from [PtCl₄]²⁻, add NH₃ stepwise. Cl⁻ has a stronger trans effect than NH₃, so the second NH₃ enters cis to the first → cis product.
• trans: start from [Pt(NH₃)₄]²⁺, add Cl⁻ stepwise. After the first Cl⁻ is in, its strong trans effect directs the second Cl⁻ to the position trans to it → trans product.

Electron-transfer (redox) mechanisms

• Outer-sphere: electron tunnels between two intact complexes; no bonds broken, coordination spheres stay separate. Fast when self-exchange rates are high.
• Inner-sphere (Taube bridged): the two metals share a bridging ligand (e.g. Cl⁻) in the transition state; electron passes through the bridge. Requires at least one substitution-labile partner to form the bridge. Taube's classic system: [Co(NH₃)₅Cl]²⁺ + [Cr(H₂O)₆]²⁺ → Cl⁻ transfers to Cr along with the electron.
• Marcus theory (one line): the rate of an outer-sphere transfer is governed by the reorganisation energy (λ) and the driving force (ΔG°); it also predicts cross-reaction rates from the two self-exchange rates.

30-second recall table