Mechanism
Associative substitution occurs by the addition of a ligand to a metal complex followed by dissociation of an original ligand. Associative pathways are not typically seen in 18-electron complexes due to requisite intermediates having more than 18 electrons. 18 electron indenyl complexes; however, have been shown to undergo substitution via associative pathways quite readily. This is attributed to the relative ease η5 to η3 rearrangement due to stabilization by the arene. This stabilization is responsible for substitution rate enhancements of about 108 for the substitution of indenyl complexes compared to the corresponding cyclopentadienyl complex.
Kinetic data supports two proposed mechanisms for associative ligand substitution. The first mechanism, proposed by Hart-Davis and Mawby, is a concerted attack by the nucleophile and η5 to η3 transition followed by loss of a ligand and a η3 to η5 transition.
In a second mechanism proposed by Basolo, η5 to η3 transitions are in rapid chemical equilibrium. The rate-limiting step occurs with the attack of the nucleophile on a η3 intermediate.
Read more about this topic: Indenyl Effect
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