Stereodynamics of ethyl group rotation in co-ordinated hexaethylbenzene: X-ray crystallographic and dynamic n.m.r. spectral studies on dicarbonyl(hexaethyl-benzene)trimethylphosphinechromium(0)

The co-ordinated hexaethylbenzene (heb) of (heb)Cr(CO)2(PMe3) adopts a 1,2,3,5-distal-4,6-proximal methyl conformation, with an additional distal methyl found as a disorder in the crystal in approximately one molecule in five; the low-temperature n.m.r. spectra of (heb)Cr(CO)2(PMe3)(31P and 13C) are satisfactorily explained by slowed ethyl group rotation resulting in multiple heb steroisomers coexisting in equilibrium and Without invoking slowed rotation about the metal-arene bond.