Reaction Mechanisms of Coordination Compounds (Substitution)

Core Idea

Ligand substitution in coordination compounds proceeds through dissociative (D), associative (A), or interchange (I) mechanisms, analogous to SN1, SN2, and concerted pathways in organic chemistry. Octahedral complexes predominantly undergo dissociative substitution, while square planar complexes favor associative substitution. The trans effect governs the selectivity of substitution in square planar complexes, and crystal field activation energy (CFAE) determines whether a complex is labile or inert.

Explainer

Understanding how coordination compounds react is as important as understanding their structure. Ligand substitution — replacing one ligand with another — is the most common reaction type for coordination compounds, and its mechanism determines the rate, selectivity, and stereochemical outcome. The mechanistic framework parallels organic chemistry but with important differences arising from the metal center's electronic structure.

Octahedral substitution reactions predominantly follow dissociative or dissociative interchange pathways. The transition state involves partial dissociation of the leaving ligand, generating a five-coordinate intermediate (or transition state) before the incoming ligand bonds. Evidence for this comes from the rate law: substitution rates for octahedral complexes typically depend on the concentration of the complex but not on the concentration of the incoming ligand, indicating that bond breaking is the rate-determining step. The identity of the incoming ligand has little effect on the rate — consistent with dissociation occurring before association. Crystal field activation energy (CFAE) provides a theoretical framework for predicting lability: complexes that lose significant CFSE upon distorting to the transition-state geometry are kinetically inert, while those with little CFSE to lose are labile.

Square planar substitution follows the opposite pattern: associative mechanisms dominate. The open coordination sites above and below the molecular plane allow a fifth ligand to approach and form a five-coordinate trigonal bipyramidal transition state. The rate law shows dependence on both the complex and incoming ligand concentrations. Most importantly, the trans effect governs selectivity: ligands with strong trans influence weaken the bond to the ligand opposite them, directing which position is substituted. This kinetic directing effect enables stereochemical control — the synthesis of cisplatin being the most celebrated example.

The Taube classification of complexes as labile or inert provides practical guidance. Labile complexes (d⁰, d¹, high-spin d⁴-d⁷, d⁹, d¹⁰) exchange ligands rapidly — typically with half-lives of seconds or less. Inert complexes (d³, low-spin d⁴-d⁶) exchange slowly, with half-lives of hours to days. Note that inert does not mean thermodynamically stable: a complex can be thermodynamically unstable but kinetically inert (it wants to react but cannot reach the products quickly). This distinction between thermodynamic stability and kinetic lability is one of the most important concepts in coordination chemistry.