HETEROBIMETALLIC INDENYL COMPLEXES - KINETICS AND MECHANISM OF SUBSTITUTION AND EXCHANGE-REACTIONS OF TRANS-[CR(CO)(3)-INDENYL-RH(CO)(2)] WITH OLEFINS

The trans coordination of the benzene ring of the indenyl-Rh(CO)(2) co mplex with tricarbonylchromium strongly enhances the rate of substitut ion of CO's with bidentate olefins, 1,5-cyclooctadiene (COD) and norbo rnadiene (NBD) (''extra-indenyl effect''). The activation parameters s uggest an associative reaction pathway assumed to proceed via the inte rmediacy of a nonisolable low-hapticity species, eta(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(C O)(3)-indenyl-Rh(COD)], 3a, and suitable accepters (hexamethylbenzene and cycloheptatriene) is markedly increased with respect to that measu red for the same reaction in the monometallic complex eta-naphthalene- Cr(CO)(3) (''extra-naphthalene effect''). These mutual effects of the Cr(CO)(3) and RhL(2) units are transmitted through the 10 pi electron indenyl framework, and the results obtained are in agreement with the existence of an haptomeric ground-state equilibrium between the two is omers trans-[Cr(CO)(3)-mu-eta(6),eta(3)-indenyl-RhL(2)], I, and trans- [Cr(CO)(3)-mu,eta(4):eta(5)-indenyl-RhL(2)], II.