Indenyl Effect - History

History

The indenyl effect is a term given by Fred Basolo to a phenomenon first reported by Adam J. Hart-Davis and Roger J. Mawby in 1969. Hart-Davis and Mawby found that the rate of conversion of (η5-C9H7)Mo(CO)3CH3 to the phosphine-substituted acetyl complex followed bimolecular kinetics. This rate law 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 corresponding reaction of tributylphosphine with (η5-C5H5)Mo(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 the SN2 replacement of CO in Rh(η5-C9H7)(CO)2 to be 108 times faster than in Rh(η5-C5H5)(CO)2. Shortly afterwards, Basolo tested the effect of the indene ligand on Mn(η5-C9H7)(CO)3, the cyclopentadienyl 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.

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