Base Catalyzed Hydrolysis: The CB Mechanism
The rates of substitution of octahedral complexes are not sensitive to the nature of the entering group, with on exception. In basic media, CoIII complexes having ligands of the type NH3, RNH2, and R2NH are sensitive to the nature of the entering group.
The base catalyzed reactions are generally much more rapid than hydrolyses in acid solution.
The rate of reaction is observed to depend on the concentrations of both the complex and the base. The mechanism involves the removal of a proton from the amine ligand, before the rate determining loss of the leaving group. The initial removal of the proton represents the rapid setting up of a pre-equilibrium, and is around 105 faster than the rate determining step.
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Base-Catalyzed Hydrolysis: The CB Mechanism The NH3 group is transiently deprotonated and this labilizes the Cl ligand. There is also stabilization of the trigonal bipyramidal intermediate by the NH2 group. |
Isomerization of Complexes
The isomerization reactions are closely related to the substitution reactions: these often provide a mechanism for isomerization. The intermediates in the dissociative substitution of octahedral complexes and associative substitution of square planar complexes are trigonal bipyramidal in shape, and the mixture of products formed in these reactions results from a rearrangement of this intermediate.
This rearrangement can also occur without the substitution taking place, with the sequence being the fission of a metal-ligand bond, the rotation of the intermediate complex, and then the reformation of the metal-ligand bond.
| The Bailar Twist |
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| The Ray- Dutt Twist |
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The first mechanism shown above, the Bailar Twist, demonstrates one way by which an octahedral complex may undergo isomerization without the breaking of a metal-ligand bond. The intermediate is a trigonal biprism with the ligands stretching along each of the vertical edges. The similar intermediate with one ligand stretching across a vertical edge, and the other two stretching across parallel horizontal edges is that in the process known as the Ray-Dutt Twist. The racemization of [Ni(en)3]2+ occurs by the Bailar Twist mechanism.
Carbonyl Insertion
When an atom belonging to a ligand other than the entering or leaving groups becomes directly involved in the reaction, the mechanism becomes more complex. Such a scenario is the reaction:
The net reaction is the insertion of the carbon monoxide ligand into the M-CH3 bond to give a carbonyl group. The mechanism involves the formation of a bond from the carbon of the methyl group to the carbon on a CO following nucleophilic attack on the CO carbon by the methyl carbon (the methyl carbon is Cδ- and the CO carbon is Cδ+). This is consistent with a negative reaction entropy, consistent with the incorporation of the free ligand into the activated complex, and also with the fact that electron withdrawing groups attached to the methyl group dramatically slow the reaction, by reducing the nucleophilicity of the methyl carbon.
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The intermediate in the methyl migration mechanism. |
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The methyl group migrates to one of the cis-CO ligands, and then the new CO ligand enters the position vacated by the methyl group.