Elementary Steps of Catalysis: Oxidative Addition and Insertion
Almost every organometallic catalytic cycle is built from the same handful of steps. Each one changes the electron count and the metal's oxidation state in a fixed way, which is what makes cycles predictable.
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
- Explains what has to be true of a metal centre before oxidative addition can occur
1 · Read
Almost every organometallic catalytic cycle is built from the same handful of steps. Each one changes the electron count and the metal oxidation state in a fixed way, which is what makes cycles predictable. To track them you must first assign oxidation numbers reliably: pure elements are zero, hydrogen is usually plus 1, oxygen is usually minus 2, and the parts must sum to the overall charge.
In oxidative addition the metal breaks a bond in a molecule such as H2 or an alkyl halide and takes both fragments onto itself. Count carefully and you will see the oxidation state rise by 2 and the electron count rise by 2. The reverse step is reductive elimination, where the oxidation state falls by 2. A sixteen electron complex meeting an aryl halide is the classic setup: the product sits at the higher oxidation state with eighteen electrons.
Migratory insertion works differently: one ligand migrates onto its neighbour ligand. The metal oxidation state stays put while the electron count drops by two, which opens a vacant site for the next incoming ligand. Recognise it by asking which ligand moved, since simple addition has no mover.
Beta hydride elimination completes the core set: a hydrogen shifts from the carbon chain to the metal, undoing an insertion. Before oxidative addition can happen at all, the centre must have room and the right low oxidation state, typically an unsaturated complex ready to accept two new fragments. Balance electrons on both sides of every step, as with half reactions, and the cycle closes predictably.
Oxidative addition adds two and two, insertion moves a ligand and frees a site, and elimination reverses the flow.
2 · Watch
Take it off screen
Where it sits
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.