beta-hydride elimination

Imagine an alkyl chain dangling from a metal. Walk one carbon out from the metal: that first carbon is the alpha carbon, and the next one along is the beta carbon. If the beta carbon is carrying a hydrogen, the metal can reach back, pluck that hydrogen off, and in the same motion let the chain fold up into an alkene that drifts off the metal. This move is beta-hydride elimination, and it is the exact reverse of an alkene inserting into a metal-hydride bond.

Step by step: a metal-alkyl with a hydrogen on its beta carbon swings that carbon-hydrogen bond up against the metal, the metal and the hydrogen briefly share a four-membered arrangement, the C-H bond breaks, and out come two products bonded to the metal — a new metal-hydride and a coordinated alkene formed from the rest of the chain. The bookkeeping is mild: the metal's oxidation state does not change, but because one alkyl ligand has split into a hydride plus an alkene, the metal needs an empty coordination site for the forming alkene to land on. That requirement is the key: a coordinatively saturated 18-electron complex with no open site cannot do beta-hydride elimination until it loses a ligand, and a metal-alkyl with no hydrogen on the beta carbon simply cannot do it at all.

Beta-hydride elimination is a double-edged sword in catalysis. On the useful side it is a chain-termination and isomerization step, and the whole basis of catalytic dehydrogenation and of how metals shuffle double bonds along a chain. On the troublesome side it is the bane of anyone trying to make a stable metal-alkyl: many simple alkyls fall apart by beta-hydride elimination almost as fast as you make them, which is why chemists prize alkyls that have no beta-hydrogen, such as methyl, neopentyl, or trimethylsilylmethyl, when they need a metal-carbon bond to survive. Knowing when this step will fire, and how to block it, is everyday strategy in organometallic chemistry.