Summary
Transition metal complexes play a pivotal role in modern organic synthesis. By leveraging partially filled d orbitals and unique electron counting methods (18-electron rule), these complexes can facilitate diverse reactions from functional group interconversions to carbon–carbon bond formations. Key mechanistic steps such as ligand dissociation–association, oxidative addition/reductive elimination, and insertion–deinsertion underpin many catalytic processes including homogeneous hydrogenation, cross-coupling, and olefin metathesis. These reactions have broad applications in pharmaceutical synthesis, natural product construction, and polymer chemistry, underscoring their significance in both academic and industrial settings.