The stereochemistry of organometallic compounds. Part V. Preferred conformations of tricarbonylmetal groups in some cis-tricarbonyl-(1-substituted indane)chromiums

The n.m.r. spectrum of tricarbonyl-(trans-1,3-dimethylindane)chromium (I; M = Cr) shows separate signals for the four aromatic protons, the separation being sufficient for the spectra to be considered first order. The aromatic regions of the spectra of other cis-tricarbonyl-(1-substituted indane)chromiums show similar properties, in contrast to the spectra of the corresponding trans-isomers. The signal separation in the case of the cis-isomers is attributed to the tricarbonylchromium residue adopting a preferred conformation in which steric interactions with the bulky cis-substituent are minimised.