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The indenyl effect is a term associated with η5 to η3 rearrangement in indenyl metal compounds that facilitates associative ligand exchange.

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 compared to cyclopentadienyl analogues.

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 η5 to η3 transition.

In a second mechanism proposed by Basolo, η5 to η3 transitions are in rapid equilibrium, the rate-limiting step occurs with the attack of the nucleophile on an η3 intermediate.

η5 to η3 Rearrangement in Other Ligands
Indenyl like effects are also observed in a number of non indenyl substituted metal complexes. In fluorenyl substituted metals, associative substitution is enhanced even further than indenyl compounds. The substitution rate of Mn(η5-C13H9)(CO)3 is about 60 times faster than that of Mn(η5-C9H7)(CO)3

Veiros conducted a study comparing the rate of substitution on [(η5-X)Mn(CO)3] where X is cyclopentadienyl, indenyl, fluorenyl, cyclohexadienyl, and 1-hydronaphthalene. Unsurprisingly, it was found that the ease of η5 to η3 haptotropic shift correlated to the strength of the Mn-X bond.



History
The indenyl effect is a term coined by Fred Basolo to a phenomena first reported by Adam J. Hart-Davis and Roger J. Mawby in 1969. In Hart-Davis and Mawby’s work, it was found that the methyl migration in Mo(η5-C9H7)(CO)3CH3 was SN2 in nature. This was attributed to the ability of the indenyl ligand to undergo a η5 to η3 haptotropic rearrangement which abets associative attack on the metal. The reaction with Mo(η5-C5H5)(CO)3CH3 was 10 times slower.

Subsequent work by Hart-Davis, Mawby, and White compared CO substitution by phosphines in Mo(η5-C9H7)(CO)3X and Mo(η5-C5H5)(CO)3X (X = Cl, Br, I) and found the cyclopentadienyl compounds to substitute by an SN1 pathway and the indenyl compounds to substitute by both SN1 and SN2 pathways. Mawby and Jones later studied the rate of CO substitution with P(OEt)3 with Fe(η5-C9H7)(CO)2I and Fe(η5-C5H5)(CO)2I and found that both occur by an SN1 pathway with the indenyl substitution occurring about 575 times faster. Hydrogenation of the arene ring in the indenyl ligand resulted in CO substitution at about half the rate of the cyclopentadienyl compound.

Work in the early 1980s by Fred Basolo found that the SN2 replacement of CO in Rh(η5-C9H7)(CO)2 to be 108 times faster than Rh(η5-C5H5)(CO)2. Shortly afterwards, Basolo tested the effect of the indene ligand on Mn(η5-C9H7)(CO)3, the cyclopetadienyl analogue of which having been shown to be inert to CO substitution. Mn(η5-C9H7)(CO)3 did undergo CO loss and was found to substitute via an SN2 mechanism.