HETEROBIMETALLIC INDENYL COMPLEXES - THE TRANS-CIS ISOMERIZATION OF CR(CO)(3)(MU,ETA-ETA-INDENYL)RH(NBD)

Reaction of trans-[Cr(CO)(3)(mu,eta:eta-indenyl)Rh(CO)(2)] at room temperature in CH2Cl2 with norbornadiene (NBD) yields trans-[Cr(CO)(3)(mu,eta:eta-indenyl)Rh(NBD)], 3. X-ray analysis for 3 shows a = 8.200(5) Angstrom, b = 11.388(5) Angstrom, c = 17.868 Angstrom,beta = 92.1 degrees, space group P2(1)/c. In contrast with the cis-[Cr(CO)(3)(mu,eta:eta-indenyl)Rh(NBD)], 2, isomer, the indenyl moiety in 3 is far less distorted, and the planes defined by [C-1, C-2, C-3] of the indenyl frame and by [C-11, C-12, C-14, C-15] of NBD are almost exactly parallel to each other, showing an undistorted coordination about the rhodium atom. Under suitable conditions (THF as solvent in the presence of the tetrafluoroborate salt of [Rh(NBD)](+), or a similar cation species) 3 undergoes a fast intramolecular rearrangement to give the isomer cis-[Cr(CO)(3)(mu,eta:eta-indenyl)Rh(NBD)], 2. The process exhibits a relatively low activation enthalpy (Delta H-double dagger = 11.7 +/- 0.9 kcal mol(-1)) and a very negative activation entropy (Delta S double dagger = -30 +/- 3 cal mol(-1) K-1).