The indenyl effect refers to an explanation for the enhanced rates of substitution exhibited by η-indenyl complexes vs the related η-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 η to η rearrangement due to stabilization by the arene.  This stabilization is responsible for substitution rate enhancements of about 10 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 η to η transition followed by loss of a ligand and a η to η transition.
In a mechanism proposed by Basolo, η and η isomers exist in rapid chemical equilibrium.  The rate-limiting step occurs with the attack of the nucleophile on a η isomer. The nature of the substituents of the allyl group can strongly affect the kinetics and regiochemistry of the nucleophilic attack.