Bis(η⁵-Pentamethylcyclopentadienyl) Complexes of Titanium, Zirconium, and Hafnium

The chemistry of organometallic derivatives of titanium and zirconium bearing two pentamethylcyclopentadienyl groups as ancillary ligands has proven to be rich and varied, whereas that for hafnium is still in its formative stages. Derivatives of permethyltitanocene, permethylzirconocene, and permethylhafnocene offer a number of advantages over their (η5-C5H5) analogs. Generally, they exhibit higher thermal stability, solubility, and crystallinity. Furthermore, the steric bulk of the pentamethylcyclopentadienyl group discourages oligomerization through singleatom bridges (H, O, N), thus rendering permethylmetallocene derivatives monomeric. Such properties have facilitated the development of a rich and diverse reaction chemistry for compounds of the group 4 transition metals with pentamethylcyclopentadienyl ancillary ligands.