Chapter 22: d-Metal Organometallic Chemistry

The 18-electron rule emerges as a central organizing principle for many stable complexes, though 16-electron species gain importance in square-planar geometries. Students encounter a diverse palette of ligands whose properties can be systematically understood through bonding theory: carbon monoxide demonstrates σ-donation paired with π-backbonding, a synergy revealed through infrared spectroscopy; phosphines exhibit tunable steric and electronic character quantified by Tolman cone angle measurements; hydrides and dihydrogen species occupy critical roles in catalytic transformations. The chapter extensively covers π-bonded ligands including alkenes, alkynes, dienes, arenes, allyls, and cyclopentadienyls, with ferrocene presented as the paradigmatic example of a sandwich metallocene architecture. Carbene ligands receive detailed treatment, distinguishing Fischer and Schrock manifolds from the increasingly important N-heterocyclic carbene family. Major compound classes include binary transition metal carbonyls, structurally diverse metallocenes, and polynuclear metal clusters rationalized through Wade's rules and isolobal analogies. The reaction chemistry section develops mechanistic understanding of fundamental organometallic transformations: ligand substitution pathways operating through associative or dissociative mechanisms, oxidative addition and reductive elimination as core catalytic steps, σ-bond metathesis in early transition metal chemistry, and migratory insertion coupled with hydride eliminations at various positions. The chapter concludes by showing how these elementary steps assemble into catalytic cycles for hydrogenation, hydroformylation, polymerization, and selective C-H activation, thereby bridging fundamental concepts to practical industrial applications.