Heterobimetallic Indenyl Complexes. Kinetics and Mechanism of Substitution and Exchange Reactions of <i>trans</i>-[Cr(CO)₃-indenyl-Rh(CO)₂] with Olefins

Abstract

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, η1-indenyl-Rh(CO)2(L2). 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 RhL2 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,η6:η3-indenyl-RhL2], I, and trans-[Cr(CO)3-μ,η4:η5-indenyl-RhL2], II