Elementary Steps of Catalysis: Oxidative Addition and Insertion
Many organometallic catalytic cycles combine oxidative addition, reductive elimination, migratory insertion and beta-hydride elimination. Tracking ligand charges and donations gives the formal oxidation-state and electron-count changes. State which ligands remain bound and whether another step occurs before assigning the final count.
What a learner can do afterwards
- States the change in oxidation state and electron count for oxidative addition and for its reverse
- Distinguishes migratory insertion from simple addition and identifies which ligand moves
- Recognises beta-hydride elimination and explains what structural feature makes it possible
- Describes common conditions that favor oxidative addition and explains how ligand dissociation can prepare a saturated complex
1 · Read
You have counted ligand donations. An X-type ligand is assigned charge -1 in ionic counting and donates 2 electrons there, or 1 in neutral counting. In conventional mononuclear two-electron oxidative addition, A-B is cleaved and both fragments bind one metal as X-type ligands. Examples include H2 and aryl halides. Reductive elimination reverses this: two bound groups couple into a molecule that leaves. Track each step separately from ligand association or dissociation.
In CO migratory insertion, a metal-bound alkyl (M-R) migrates onto coordinated CO, forming an acyl (M-C(O)R). The reacting ligands need a suitable arrangement, commonly cis, meaning adjacent coordination positions. One X-type alkyl and one 2-electron CO become one X-type acyl. Before another ligand binds, the metal oxidation state is unchanged and the count falls by 2. Direct association adds a ligand without this ligand-to-ligand bond formation.
Suppose a 16-electron M(II) complex undergoes two-electron oxidative addition with other ligands unchanged. Two new X-type ligands donate 4 electrons in ionic counting, but M(IV) has 2 fewer d electrons than M(II). The net count increases by 2 to 18. Reductive elimination reverses both changes: oxidation state falls by 2 and count falls by 2.
Many two-electron oxidative additions are favored by low-valent, electron-rich metal centres with accessible coordination capacity. A saturated 18-electron precursor may first lose a ligand, giving a reactive species that then adds A-B. These are common conditions, not a universal rule that every starting complex must already be unsaturated. Substrate, metal, geometry and mechanism also matter.
Beta-hydride elimination moves H from a metal alkyl's beta carbon to the metal, forming an alkene. The alpha carbon binds M; the beta carbon is next. It normally needs a beta C-H bond, an accessible site and syn-coplanar M-C-C-H geometry: these atoms lie in one plane, with M and H on the same side of C-C. If the alkene stays bound, oxidation state is unchanged and the count rises by 2. This reverses alkene insertion into M-H, not CO insertion.
CO insertion can leave a coordination position available, but solvent or another ligand may occupy it before the product is observed. A 2-electron donor then restores the count lost in insertion. Follow the actual ligand inventory: a low count alone does not prove a geometrically accessible site. A trapped product with no free site need not mean the insertion analysis was mistaken.
State the step and ligand inventory. Two-electron OA and RE change oxidation state; insertion does not. Check beta hydrogens, geometry and any later ligand binding.
Original lesson, questions and flashcards on formal organometallic reaction steps, written for this learning collection.
Two original readings on oxidative-addition bookkeeping and insertion followed by ligand association, written for this learning collection.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.