of η6- and η5-fluorenylchromium tricarbonyl anions (IXa, IXb) is proved using crossover experiments. The rearrangement IXa ⇌ IXb in the presence of 9-methylfluorenyl anion does not yield mixed products, i.e. η6- or η5-9-methylfluorenylchromium tricarbonyl anions. Similarly, under conditions of equilibrium between η6- and η5-(2,7-dideuterofluorenyl)chromiumtricarbonyl anions (XIIa, XIIb) in the presence