Indenyl effect

In organometallic chemistry, the indenyl effect refers to the enhanced rates of substitution displayed by η⁵-indenyl complexes vs the related η⁵-cyclopentadienyl complexes.

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 the requisite intermediates having more than 18 electrons associated with the metal atom. 18 electron indenyl complexes; however, have been shown to undergo substitution via associative pathways quite readily. This is attributed to the relative ease of 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 support 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.

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