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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.

Reaction of the potassium salt of [(eta6-indenyl)Cr(CO)3] at -30-degrees-C in THF with [Rh(Cl)NBD]2(NBD = norbornadiene) yields cis-[Cr(CO)3(indenyl)] Rh-(NBD), 2. Bubbling CO through a CH2Cl2 solution of 2 produces cis-[Cr(CO)3(indenyl)]Rh(CO)2, 3 in quantitative yield. X-ray analyses show for indenyl-Rh-(NBD), 1: a = 17.506(8) angstrom, b = 11.427(6) angstrom, c = 5.979(4) angstrom, beta = 90.8(2)degrees, space group P2(1)/n. For 2: a = 11.098(6) angstrom, b 17.684(8) angstrom, c = 8.253(5) angstrom, beta = 99.7(2)degrees, space group P2(1)/a. For 3: a = 16.558(8) angstrom, b 12.483(8) angstrom, c = 6.629(4) angstrom, space group P2(1)2(1)2(1). Both 2 and 3 have a doubly-bent indenyl moiety and show strongly distorted coordination modes toward the indenyl ligand of both metals. The driving force leading to such highly distorted structures probably arises mainly from a metal-metal interaction

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