In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type where R is a branched alkyl and n = 3 or 4. Representative examples are the tetraisopropyl derivative and the tris(tert-butyl) derivative . These ligands are so large that their complexes behave differently from the analogues derived from pentamethylcyclopentadienyl, itself a bulky ligand.
Because they cannot closely approach the metal, these bulky ligands can stabilize high spin complexes, such as (C<sub>5</sub>H<sub>2</sub><sup>t</sup>Bu<sub>3</sub>)<sub>2</sub>Fe<sub>2</sub>I<sub>2</sub>. These large ligands stabilize highly unsaturated derivatives such as (C<sub>5</sub>H<sub>2</sub><sup>t</sup>Bu<sub>3</sub>)<sub>2</sub>Fe<sub>2</sub>N<sub>2</sub>.
The (tert-butyl)cyclopentadiene is prepared by alkylation of cyclopentadiene with tert-butyl bromide in the presence of sodium hydride and dibenzo-18-crown-6. The intermediate in this synthesis is di-tert-butylcyclopentadiene. This compound is conveniently prepared by alkylation of cyclobutadiene with tert-butyl bromide under phase-transfer conditions.
Illustrative of the unusual complexes made possible with these bulky ligands is molecular iron nitrido complex (<sup>t</sup>Bu<sub>3</sub>C<sub>5</sub>H<sub>2</sub>)<sub>2</sub>Fe<sub>2</sub>N<sub>2</sub>. In contrast to (C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Ir<sub>2</sub>Cl<sub>4</sub>, (<sup>t</sup>Bu<sub>3</sub>C<sub>5</sub>H<sub>2</sub>)IrCl<sub>2</sub> is monomeric.
Less bulky ligand are of the type where R is a branched alkyl. Examples include the di-tert-butyl and related trimethylsilyl derivatives. One such complex is .