Cyclopentadienylthallium, also known as thallium cyclopentadienide, is an organothallium compound with formula C<sub>5</sub>H<sub>5</sub>Tl. This light yellow solid is insoluble in most organic solvents, but sublimes readily. It is used as a precursor to transition metal and main group cyclopentadienyl complexes, as well as organic cyclopentadiene derivatives.
Cyclopentadienylthallium is prepared by the reaction of thallium(I) sulfate, sodium hydroxide, and cyclopentadiene:
The compound adopts a polymeric structure, consisting of infinite chains of bent metallocenes. The Tl---Tl---Tl angles are 130ð. Upon sublimation, the polymer cracks into monomers of C<sub>5v</sub> symmetry.
Compared to other cyclopentadienyl (Cp) transfer reagents, such as cyclopentadienyl sodium, CpMgBr and Cp<sub>2</sub>Mg, cyclopentadienylthallium is less air sensitive. It is also much less of a reducing agent.