ernst otto fischer in 1962 discovered tetracyclopentadienyluranium cpu by reaction of nacp with ucl (6% yield) as a compound stable in air as a solid but not in solution. a zero molecular dipole moment and ir spectroscopy revealed that it was also a sandwich compound with uranium in a tetrahedral molecular geometry. in 1970, fischer added cpu to the list of known organouranium compounds by reduction of cpu with elemental uranium.