Substituted cyclopentadienyl compounds: IV. NMR spectra of chloro-substituted (η 5-cyclopentadienyl)-rhodium-(η 4-diene) compounds and the crystal structure of (η 5-chlorocyclopentadienyl)-(η 4-2,3-dimethoxybutadiene)rhodium ( I)

The molecular structure of (η 5-C 5H 4Cl)Rh( η 4-C 4H 4(OMe) 2) ( I) has been determined. The orientation of the ligands is one in which the cyclopentadienyl ring substituent bond lies in a mirror plane that bisects the two terminal carbon-carbon bonds of the butadiene ligand. This conformation is associated with population of a cyclopentadienyl rotamer of η 4-diolefin-η 1-alkyl character. Extended Hückel molecular orbital calculations show that this rotamer is preferred to the alternative η 3-allyl-η 2-ene form in I. The 1H NMR data for I and related compounds suggest that the metal-coupling patterns reflect the variation in charge density within the substituted cyclopentadienyl ring.