What is Bulky cyclopentadienyl ligands?

In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type C5H5−nRn where R is a branched alkyl and n = 3 or 4. Representative examples are the tetraisopropyl derivative C5HPr4 and the tris derivative 1,2,4-C5H2Bu3. These ligands are so large that their complexes behave differently from the pentamethylcyclopentadienyl analogues. Because they cannot closely approach the metal, these bulky ligands stabilize high spin complexes, such as 2Fe2I2. These large ligands stabilize highly unsaturated derivatives such as 2Fe2N2.