MULTIPLE FLUXIONAL PROCESSES IN CHIRAL (PENTAPHENYLCYCLOPENTADIENYL)IRON COMPLEXES - X-RAY CRYSTAL-STRUCTURE OF (C5PH5)FE(CO)(PME3)(HC=O) AND THE BARRIERS TO ARYL, PHOSPHINE, AND TRIPODAL ROTATION IN (C5PH5)FE(CO)(PME2PH)C(O)ET

The X-ray crystal structure of (C5Ph5)Fe(CO)(PMe3)(HC=O) (5) reveals t hat the phenyl rings adopt a paddle-wheel orientation with an average tilt of 50-degrees relative to the plane of the five-membered ring. 5 crystallizes in the monoclinic space group I2/a with a = 24.159(8) ang strom, b = 12.868(7) angstrom, c = 25.861(9) angstrom, beta = 93.95(5) degrees, V = 8021(1) angstrom3, and Z = 8. C-13 and P-31 variable-temp erature NMR spectra of the sterically crowded molecule (C5Ph5)Fe(CO)(P PhMe2)C(O)Et reveal the presence of two fluxional processes. When rota tion of the chiral tripod becomes slow on the NMR time scale, the 5-fo ld degeneracy of the cyclopentadienyl ring carbons is split. Moreover, slowed rotation of either the peripheral phenyl rings or the phosphin e ligand can give rise to a mixture of diastereomers. The barriers for these two processes (8.7 +/- 0.3 and 11.7 +/- 0.3 kcal mol-1, respect ively) demonstrate that the two types of fluxional behavior are not co rrelated.