reductive elimination

If oxidative addition is the metal grabbing two pieces, reductive elimination is the metal handing them back joined together — the moment a catalyst finally lets go of the product it has built. Two groups that sit next to each other on the metal couple up, leave as a single new molecule with a fresh bond between them, and the metal is left lighter, lower in oxidation state, and ready to start again.

It is the exact microscopic reverse of oxidative addition. Two adjacent ligands, X and Y, that are bonded to the same metal join to form X-Y, which departs. The bookkeeping mirrors oxidative addition perfectly: the metal's coordination number falls by two, its formal oxidation state drops by two, and its valence electron count falls by two. For the two groups to couple they must be cis, sitting side by side, since a new bond can only form between neighbours; trans groups must first rearrange. Reductive elimination tends to be favoured when the metal is electron-poor or crowded, and when the two leaving groups are a good match for bonding to each other, such as two carbon groups, or a carbon and a hydrogen.

This step matters because it is the product-forming, catalyst-releasing climax of a catalytic cycle. In cross-coupling, after the palladium has gathered two organic fragments onto itself, reductive elimination welds them into the new carbon-carbon bond and spits out the coupled product, regenerating palladium(0) to begin the next turn. In hydrogenation, reductive elimination of a hydride and an alkyl releases the saturated alkane. Because it lowers oxidation state and frees a site, it pairs naturally with oxidative addition to form a complete loop. The honest point is that whether reductive elimination or its reverse wins is a balance of thermodynamics and the nature of the groups; bulky ligands that crowd the metal often help by pushing the two groups together to eliminate.