the development of organouranium compounds started in world war ii when the manhattan project required volatile uranium compounds for u/u isotope separation. for example, henry gilman attempted to synthesize compounds like tetramethyluranium and others worked on uranium metal carbonyls but none of the efforts met success due to organouranium instability. after the discovery of ferrocene in 1951, todd reynolds and geoffrey wilkinson in 1956 synthesized the uranium metallocene cpucl from sodium cyclopentadienide and uranium tetrachloride as a stable but extremely air-sensitive compound. in it the u-cl bond is an ionic bond and the uranium bonds with the three cyclopentadienyl ligands are covalent of the type found in sandwich compounds with involvement of the uranium 5f atomic orbitals.