A transition-metal complex in which a metal atom or ion is ‘sandwiched’ between two rings of atoms. Ferrocene was the first such compound to be prepared, having two parallel cyclopentadienyl rings with an iron ion between them. In such compounds (also known as metallocenes) the metal coordinates to the pi electron system of the ring, rather than to individual atoms. A wide variety of these compounds are known, having five-, six-, seven-, or eight-membered rings and involving such metals as Cr, Mn, Co, Ni, and Fe. Other similar compounds are known. A multidecker sandwich has three or more parallel rings with metal atoms between them. In a bent sandwich, the rings are not parallel. A half sandwich (or piano stool) has one ring, with single ligands on the other side of the metal. The theoretical analysis of sandwich compounds frequently makes use of electron counting rules.