Stereochemistry and stereodynamics of η ⁶ -hexaethylbenzene transition metal complexes. Crystal and molecular structure of dicarbonyl(η ⁶ -hexaethylbenzene)(η ² -maleic anhydride)molybdenum( 0 )

The variable-temperature 90.1-MHz 13C-{1H} and/or 145.87-MHz 31P-{1H} n.m.r, spectra of dicarbonyl(η6- hexaethylbenzene)(thiocarbonyl)chromium(0), (4), dicarbonyl(η6-hexaethylbenzene)(trimethylphosphine)chromium(0), (5), dicarbonyl(η6-hexaethylbenzene)(triethylphosphine)chromium(0), (6), dicarbonyl(η6-hexaethylbenzene)(triethylphosphine)molybdenum(0), (7), dicarbonyl(η6-hexaethylbenzene)(triphenyl phosphite)chromium(0), (9), dicarbonyl(η6-hexaethylbenzene)(η2-malefic anhydride)chromium(0), (10), dicarbonyl(η6-hexaethylbenzene)(η2-maleic anhydride)molybdenum(0), (11), and (η5-cyclopentadienyl)(η6-hexaethylbenzene)iron(II) hexafluorophosphate, (12), have been observed and subjected to lineshape analysis. In a number of cases, multiple heb stereoisomers are found to coexist in equilibrium in solution at low temperatures. For all the hexaethylbenzene (heb) complexes examined, the n.m.r. spectra can be satisfactorily analysed without the need to invoke hindered rotation about the metal–arene bond and coalescence phenomena observed in the spectra are attributed to the onset of rapid rotation of the ethyl groups about the benzene ring and heb stereoisomerisation. Barriers to site exchange (ΔG‡) of the ethyl groups are reported for a number of the heb complexes. The crystal and molecular structure for (11) has been determined and the co-ordinated heb has been found to adopt a 1,3,5-distal-2,4,6-proximal methyl conformation.