Compounds in which a metal atom is located between the faces of two parallel and planar (or nearly so) ring structures, e.g. bis(η 5-cyclopentadienyl)iron (ferrocene), dibenzenechromium.
See also: metallocenes
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1364 (https://doi.org/10.1351/pac199567081307)