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The trans coordination of the benzene ring of the indenyl-Rh(CO) 2 complex with tricarbonylchromium strongly enhances the rate of substitution of CO's with bidentate olefins, 1,5-cyclooctadiene (COD) and norbornadiene (NBD) ("extra-indenyl effect"). The activation parameters suggest an associative reaction pathway assumed to proceed via the intermediacy of a nonisolable low-hapticity species, η 1 -indenyl-Rh(CO) 2 (L 2 ). In addition, the rate of exchange of the Cr(CO) 3 group of the complexes trans-[Cr(CO) 3 -indenyl-Rh(CO) 2 ], 3, and trans-[Cr(CO) 3 -indenyl-Rh(COD)], 3a, and suitable acceptors (hexamethylbenzene and cycloheptatriene) is markedly increased with respect to that measured for the same reaction in the monometallic complex η-naphthalene-Cr(CO) 3 ("extra-naphthalene effect"). These mutual effects of the Cr(CO) 3 and RhL 2 units are transmitted through the 10 π electron indenyl framework, and the results obtained are in agreement with the existence of an haptomeric ground-state equilibrium between the two isomers trans-[Cr(CO) 3 -µ,η 6 :η 3 -indenyl-RhL 2 ], I, and trans-[Cr(CO) 3 -µ,η 4 :η 5 -indenyl-RhL 2 ], II.

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