metallocenes

Organometallic coordination compounds in which one atom of a transition metal such as iron, ruthenium or osmium is bonded to and only to the face of two cyclopentadienyl [η5-(C5H5)] ligands which lie in parallel planes. The term should not be used for analogues having rings other than cyclopentadienyl as ligands.
M03865
Source:
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1349