The indenyl analogues of ferrocene, which is orange, and cobaltocenium cation were first reported by Pauson and Wilkinson.  The cobalt derivative is a poorer reductant than cobaltocene.
The indenyl effect was discovered by Hart-Davis and Mawby in 1969 through studies on the conversion of (η-CH)Mo(CO)CH to the phosphine-substituted acetyl complex, which follows bimolecular kinetics. This rate law was attributed to the haptotropic rearrangement of the indenyl ligand from η to η.  The corresponding reaction of tributylphosphine with (η-CH)Mo(CO)CH was 10 x slower. The term indenyl effect was coined by Fred Basolo.
Subsequent work by Hart-Davis, Mawby, and White compared CO substitution by phosphines in Mo(η-CH)(CO)X and Mo(η-CH)(CO)X (X = Cl, Br, I) and found the cyclopentadienyl compounds to substitute by an S1 pathway and the indenyl compounds to substitute by both S1 and S2 pathways.  Mawby and Jones later studied the rate of CO substitution with P(OEt) with Fe(η-CH)(CO)I and Fe(η-CH)(CO)I and found that both occur by an S1 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 Basolo found the S2 replacement of CO in Rh(η-CH)(CO) to be 10 times faster than in Rh(η-CH)(CO).  Shortly afterwards, Basolo tested the effect of the indene ligand on Mn(η-CH)(CO), the cyclopentadienyl analogue of which having been shown to be inert to CO substitution.  Mn(η-CH)(CO) did undergo CO loss and was found to substitute via an S2 mechanism.