Oxidative addition, reductive elimination and insertion

A transformation name does not specify every pathway

Oxidative addition and reductive elimination organize changes in metal–ligand connectivity and formal oxidation assignment. Oxidative addition can proceed through different concerted, polar or radical pathways. A common single-metal two-electron example changes metal oxidation state by +2, but the reaction name alone does not establish every intermediate, rate law or geometry.

For each selected example here, first state the entity charge, ligand assignments and retained ligands. Then compare atoms, formal metal state and the same valence-count convention. A formal increase in metal oxidation state need not mean that free electrons have escaped the whole reacting system; the electrons are reassigned within a conserved molecular net.

An intact substrate can become two assigned ligands

Choose the formal net IrCl(CO)(PPh₃)₂+H₂→IrClH₂(CO)(PPh₃)₂, retaining the other ligands. With Cl⁻ and two product H⁻ assignments, the metal changes Ir+1, d⁸→Ir+3, d⁶. Ionic ligand donation grows by four from the two hydrides, while the metal d count decreases by two. The valence total therefore changes sixteen→eighteen.

In the stated one-donor-site-per-ligand drawing, coordination sites change four→six, and the whole entity remains neutral. Two H nuclei are conserved. This is the selected single-metal two-electron bookkeeping, not a universal statement that every oxidative addition has that ligand inventory, geometry or mechanism. An intact H₂ ligand before cleavage is a separate connectivity state.

Reductive elimination reverses a selected connectivity change

In the symbolic model PdL₂(CH₃)(H)→PdL₂+CH₄, take L as a neutral monodentate two-electron donor, methyl −1 and hydride −1. The starting neutral entity has Pd+2, d⁸ and sixteen electrons. The returned PdL₂ model has Pd0, d¹⁰ and fourteen. One C and four H atoms appear as methane; both retained L ligands remain in the metal pool.

The two X-type ligand assignments combine into a neutral molecule. Metal d count increases by two while ligand donation decreases by four, giving a net decrease of two in this selected metal valence count. Suitable disposition and actual kinetic viability require evidence. The formal net does not prove that every PdL₂ species is isolable or that methane formation proceeds at a particular rate.

Migration and incoming CO are different steps

For a selected methylmanganese model, CH₃Mn(CO)₅ has Mn+1, d⁶ and eighteen electrons. Migration between its bound methyl and one bound CO gives a C-bound acyl entity Mn(CO)₄[C(O)CH₃]. Mn remains +1; the former neutral CO donor and methyl donor combine into one assigned acyl −1 donor. The immediate formal valence count is sixteen.

The initial and immediate-product inventories are Mn₁C₆O₅H₃. No external carbon has been added in that step. A separate incoming free CO can then associate, restoring an eighteen-electron count and adding one C and one O to the metal entity. Calling the first step CO insertion does not by itself establish that an unbound CO collided directly into the metal–alkyl bond.

A suitable adjacent arrangement of the reacting bound groups is important in the selected migration description, but does not alone prove a rate or a particular intermediate lifetime. Different acyl binding, bridging or spectator-ligand changes require their own ledgers. The diagrams and counts here deliberately retain the C-bound acyl assumption.

Insertion into a metal–H interaction can conserve oxidation state

Choose neutral model RhClL₂H₂(η²-C₂H₄), with two hydrides and a bound neutral alkene. Rh is +3, d⁶ and its count is eighteen. Migration of one hydride to the bound ethene creates an ethyl −1 ligand, giving RhClL₂H(C₂H₅) with Rh still +3 and sixteen electrons. The reacting H⁻ plus C₂H₄ pool becomes C₂H₅⁻; the second hydride remains.

The selected ligand-site count changes six→five, while direct metal–atom contacts change seven→five because the initial alkene has two carbon contacts. These different counts must not be silently equated. A new ligand could subsequently bind, but it is not part of this immediate insertion ledger.

The reverse β-hydride elimination cleaves a suitably accessible C–H interaction at the carbon next to the metal-bound carbon, giving bound hydride and alkene in the selected model. It requires appropriate geometry and accessible adjacent coordination space under the chosen mechanism. It does not necessarily release free alkene immediately. Absence of a β H blocks this simple pathway, while alternative pathways and actual rates need additional evidence.

Worked examples

Try it yourself

1. In the selected Ir H₂ addition, metal d count falls by two and donation rises by four. What is the net valence-count change?

   Show the answer and reasoning

   An increase of two electrons.

   −2+4=+2 under the same ionic convention; the whole system has not created two electrons.

2. Before external CO association, how many C and O atoms are in each selected methyl-Mn and immediate acyl-Mn entity?

   Show the answer and reasoning

   Six C and five O atoms in each.

   The methyl supplies one C, and the five original CO units supply five C and five O; migration reorganizes that pool.

3. Why does the simple β-hydride-elimination model require more than the mere presence of a C–H bond somewhere in the ligand?

   Show the answer and reasoning

   The H must be an accessible β H with suitable local geometry and coordination space.

   A remote C–H bond or an incompatible arrangement does not satisfy this selected elementary-class description.

4. Does an oxidative-addition name prove a concerted molecular pathway?

   Show the answer and reasoning

   No.

   The reaction class can include different polar, concerted or radical mechanisms; evidence must discriminate them.

What to carry forward

• State the retained ligands and charge before comparing counts.
• Reaction classes do not specify every pathway.
• Keep bound-ligand migration separate from external association.
• Conserve atoms alongside formal electron assignments.
• Distinguish ligand functions, contacts and accessible β-H geometry.