Chapter 21: Coordination Chemistry: Reactions of Complexes

Ligand substitution reactions, where one ligand displaces another from a metal center, are classified mechanistically as dissociative, associative, or interchange pathways, each characterized by distinct activation barriers and rate-determining steps. The lability of complexes depends critically on electronic configuration: s-block and d¹⁰ metal ions display rapid exchange kinetics, while d³ and low-spin d⁶ complexes exhibit kinetic inertness due to ligand-field stabilization effects. Square-planar complexes of Pt(II), Pd(II), and Au(III) receive detailed attention, with emphasis on quantifying nucleophilicity through reactivity parameters, explaining trans effects through strong σ-donor or π-accepting ligand properties, and describing trigonal-bipyramidal transition states that govern stereochemical outcomes. Octahedral complexes predominantly undergo substitution via interchange mechanisms, proceeding through Eigen-Wilkins encounter complexes and often facilitated by base hydrolysis of coordinated water or ammonia ligands, with five-coordinate intermediates determining stereochemical retention. Redox mechanisms are divided into inner-sphere pathways, where bridging ligands mediate electron transfer between metal centers as exemplified by the Taube mechanism, and outer-sphere processes involving direct electron tunneling without ligand rearrangement. Marcus theory provides quantitative treatment of outer-sphere electron transfer through reorganization energy concepts and self-exchange rate constants, establishing relationships for cross-reactions between metal redox couples. Photochemical transformations arise when light absorption excites electrons to higher energy levels, generating new reaction pathways including ligand dissociation from metal carbonyls, photoisomerization via d-d transitions, and photoredox chemistry involving charge-transfer excited states, with illustrative examples in chromium ammines, ruthenium bipyridyl complexes, and polynuclear metal-metal bonded systems.