Types of Mechanism for Substitution Reactions

The mechanism of a reaction can be discussed in different ways. The sequence of elementary steps by which the reaction takes place is known as the stoichiometric mechanism, and the details of the activation process and formation of the activated complex in the rate determining step are known as the intimate mechanism.

The Stoichiometric Mechanism

Investigation of the dependence of the rate of reaction on concentration of the reactants leads to the rate law governing the reaction. Any specie appearing in the rate law plays some role in the formation or reaction of the activated complex, and the mechanism of the reaction must reflect this. There are three possible mechanisms, dissociative, D, interchange, I, and associative, A.

The Dissociative Mechanism, D: This involves a step in which an intermediate of reduced coordination number is formed after the departure of the leaving group.
The Interchange Mechanism, I: This does not involve the formation of an intermediate species. The entering group and leaving group exchange in a single step involving an activated complex only.
The Associative Mechanism, A: This involves a step in which an intermediate of increased coordination number is formed after the incoming of the entering group.

The Intimate Mechanism

When considering the intimate mechanism, it is useful to distinguish between two types. If the rate constant for the formation of the activated complex depends on the identity of the entering group, Y, then there must be significant [X-M] to Y bonding in the activated complex. If the rate constant is independent of Y, then the reaction is controlled by the fission of the M-X bond. The intimate processes can therefore be classified as to whether they depend on the identity of Y or not, and, as such, they are associative, a, or dissociative, d.

Associative reaction, a: This is identified from a rate constant which is dependent on the entering group. Examples include the reactions of square planar complexes.

Dissociative reaction, d: This is identified from a rate constant which is independent of the entering group. Examples include the substitution of a water molecule by another ligand in a hydrated metal ion (see below).

Dissociative and associative mechanism can be compared with the S_N1 and S_N2 mechanisms in organic chemistry, but they refer only to the intimate process.

The overall mechanism

If both the stoichiometric and intimate mechanisms are known, then the overall labeling scheme may be used: A, I_a, I_d, and D are the total range of possibilities.

General Substitution Reactions of Octahedral Complexes

Studies of reactions of octahedral complexes have been largely limited to two types of reactions: the replacement of a coordinated solvent, ie. water, molecule, and solvolysis, or hydrolysis, which is the replacement of one of the complexed ligands with a water molecule.

Replacement of coordinated water molecule
Hydrolyis: coordination of a water molecule

There are several important observations about these reactions, and they can be used to predict the mechanism of the reaction.

General Reaction
Observation	Implication
Rates quite similar to water exchange rates	H2O dissociation is the important factor
Rate increases with charge on anion	The intermediate is an outer sphere complex
Rate unaffected by the nucleophilicity, basicity of the entering group	A and Ia paths thus unlikely
Rate strongly dependent on M	Suggests D or Id mechanism
Kinetics are second order	Says nothing about the mechanism
Intermediates are not detected	(they are detected in a few cases)

The implications of the observations suggest that this reaction proceeds by the I_d mechanism:

The Id Mechanism
A weak outer sphere complex is formed first, with the entering group, X, near the initial complex, [M(OH2)6]. Then the M-OH2 bond begins to break and at the same time, the entering group becomes weakly coordinated to the metal center. The bond to the leaving group, OH2, then breaks completely as the M-X bond forms.

Stereochemistry of Substitution of Octahedral complexes

When the reaction proceeds with an I_d or D mechanism, a range of products may be generated when the spectator ligands are not all the same. There is sometimes a mix of products, and the nature of the mixture depends on the ligands involved.

The range of products can be predicted by considering the possible mechanisms.

Generation of a mixture of products on substitution of an octahedral complex via the D or Id mechanism
The reaction proceeds via the square based pyramid intermediate with retention of symmetry, or through the trigonal bipyramid intermediate with a mixture of products.

When the reaction shown is that for the substitution of trans-[Co(en)₂ACl]²⁺, it forms a mixture of cis- and trans-[Co(en)₂AY]²⁺ products. It should be noted that in this case, the cis-products from reactions (2) and (3) are not the same: the resulting molecule is chiral and there are two, enantiomeric, cis-products.

The ratio of products depends  on the site and nature of the ligand A. When A is cis- to the leaving group, X, the product is always cis- as well. When A is trans- to the leaving group, the product distribution is:

When the trans-group, A, is a good π-donor ligand, isomerization is favoured due to the ability of the π-bonding to stabilize a trigonal bipyramid intermediate.