Fluorenyl derivatives of early transition elements: a synthetic and structural study

The tetracarbonyls of niobium(I) and tantalum(I), M(η5-9-phenylfluorenyl)(CO)4, have been prepared from the dinuclear tetracarbonyl anions [M2(μ-Cl)3(CO)8]− and Li(9-phenylfluorenyl). A slightly distorted four-legged piano stool geometry, due to the presence of the sterically demanding phenyl substituent, has been found in the case of the niobium derivative. The crystal and molecular structures of Zr(fluorenyl)2Me2 and of the 9-phenyl-substituted derivative have been solved. The structure of Zr(fluorenyl)2Me2 is similar to that of the known dichloride, Zr(fluorenyl)2Cl2, where fluorenyl ligands of different hapticity are present. On the other hand, the use of the more sterically demanding phenyl-substituted fluorenyl ligand in Zr(η5-9-phenylfluorenyl)2Me2, induces the fluorenyl ligands to be symmetrically and pentahapto-coordinated to zirconium.