Ligand Substitution and Electron-Transfer Mechanisms

The reactions of coordination complexes proceed by characteristic pathways—associative or dissociative substitution and inner- or outer-sphere electron transfer—that link kinetics to structure and electronic configuration.

Definition

This topic concerns the mechanisms by which ligands are replaced at a metal centre and by which electrons are transferred between metal complexes, together with the kinetic factors—pathway, geometry, and electronic structure—that control their rates.

Scope

This topic covers the kinetics and mechanisms of reactions at metal centres: associative, dissociative, and interchange ligand-substitution pathways; the lability and inertness of complexes in terms of d-electron configuration and crystal-field activation energy; the trans effect in square-planar substitution; and inner-sphere versus outer-sphere electron transfer, including the Marcus theory that predicts redox rates. It builds on the thermodynamic stability covered elsewhere by treating reaction rates rather than equilibria.

Core questions

• Does a substitution proceed by an associative or a dissociative pathway?
• Why are some complexes kinetically inert while others are labile?
• What is the trans effect and how does it direct square-planar substitution?
• How do inner-sphere and outer-sphere mechanisms differ, and what sets electron-transfer rates?

Key concepts

• Associative and dissociative pathways
• Lability and inertness
• The trans effect
• Inner-sphere electron transfer
• Outer-sphere electron transfer
• Reorganization energy

Key theories

Associative, dissociative, and interchange substitution

Ligand exchange can occur by bond making before bond breaking (associative), bond breaking first (dissociative), or a concerted interchange, with the operative pathway diagnosed from rate laws and activation parameters.

Inner-sphere and outer-sphere electron transfer

Taube showed that electron transfer can proceed through a bridging ligand shared between the two metals (inner-sphere) or without any shared ligand (outer-sphere), a distinction established by tracing atom transfer.

Marcus theory of electron transfer

Marcus related the rate of outer-sphere electron transfer to the reaction driving force and the reorganization energy of the surroundings, predicting rate trends and the inverted region.

Mechanisms

In inner-sphere transfer a bridging ligand momentarily links the two metals and may be transferred along with the electron, whereas in outer-sphere transfer the electron tunnels between intact coordination spheres at a rate set by the system's reorganization energy and driving force.

Clinical relevance

These mechanisms underpin biological electron-transport chains, the action of redox catalysts and metalloenzymes, the stability of pharmaceutical metal complexes, and corrosion and electrochemical processes.

History

Basolo and Pearson systematized the kinetics of inorganic substitution in the 1950s. Taube's labelling experiments distinguished inner- and outer-sphere electron transfer, work recognized by the 1983 Nobel Prize, and Marcus's theory, honoured in 1992, gave the quantitative framework for electron-transfer rates.

Key figures

• Henry Taube
• Rudolph Marcus
• Fred Basolo
• Ralph Pearson

Seminal works

• taube1953
• marcus1956
• weller2018

Frequently asked questions

What makes a transition-metal complex kinetically inert?

Inertness usually arises from electronic configurations that lose large amounts of crystal-field stabilization energy in the transition state, such as low-spin d6 and d3 octahedral ions, which raises the activation barrier and slows ligand exchange even when the complex is thermodynamically reactive.

How did chemists prove the inner-sphere mechanism?

Taube used a reaction in which a chloride ligand was transferred from a substitution-inert oxidant to the reduced product; finding the chloride on the new complex demonstrated that the two metals had shared a bridging ligand during electron transfer.