a, ß, y, and o Eliminations Elimination reactions are just the reverse of migratory insertion reactions. The various common elimination reactions are : H B-hydride elimination a-hydride elimination M- carbonyl elimination or decarbonylation > No change in formal oxidation state (exception: alkylidenes) V Must have an empty orbital that is cisoidal to the group doing an elimination reaction on. Alternatively, a cisoidal labile ligand that can easily dissociate to open up an empty orbital. V An 18e- complex has to lose a ligand to open up a site for elimination. In each case the addition of excess ligand inhibits the reaction by quenching the open site. Eliminations: examples * One of the hardest elimination reactions is the breaking of o- C-C bond. migratory insertion 8* H H. CH3 M-CH3 -CH3 But the reverse methyl elimination rxn is very difficult methyl elimination H Very Difficult! CH, M- CH, CH, rotation of C-C bond to move CH, group away from metal to avoid steric effects M-H Bhydride elimination One reason for this is that the C-C o-bond is surrounded by more reactive C-H bonds that short-circuit the attack on the C-C bond and can instead give a B-hydride elimination. The directed nature of the sp» hybridized C-C o-bond also makes overlap with the empty metal orbital quite difficult.