SUBSTITUTED CYCLOPENTADIENYL COMPOUNDS .3. NMR AND MOLECULAR-ORBITAL STUDY OF CONFORMATIONAL PREFERENCES IN SOME SUBSTITUTED (ETA-5-CYCLOPENTADIENYL)-RHODIUM(ETA-4-DIENE) COMPOUNDS AND THE CRYSTAL-STRUCTURE OF (ETA-5-METHANOYLCYCLOPENTADIENYL)-(ETA-4-2,3-DIMETHOXYBUTADIENE)RHODIUM(I)

The crystal structure of (eta-C5H4CHO)Rh(eta74-C4H4(OMe)2) (I) has been determined by X-ray diffraction. The preferred orientation of the ligands is one in which the cyclopentadienyl ring substituent bond bisects the internal carbon-carbon bond of the counter diene ligand. Related compounds for which four separate ring resonances appear have been analysed by nOe difference NMR spectroscopy and spectral simulation. The results show that the preferred relative orientation of ligands in solution is similar to that revealed by crystallographic studies. CNDO/U molecular orbital calculations have been performed on several of these compounds. The results are consistent with experimental findings and show that there is a preference for a rotamer in which the ring has a tendency towards the allyl-ene geometry. Carbon-13 NMR studies show that the ratios of the diene carbon chemical shifts and metal-diene coupling constants reflect the relative importance of retrodative bonding.