The parent metallacyclopentadiene, or metallole, has the formula LM(CH). Most arise from the coupling of two alkynes at a low valent metal centers such as derivatives of Co(I) and Zr(II).  Late metal derivatives (Co, Ni) are intermediates in the metal-catalysed trimerization of alkynes to arenes.  Early metal derivatives, i.e. derivatives of Ti and Zr, are used stoichiometrically. For example, the zirconacyclopentadiene CpZrCMe is a useful carrier for CMe. Some of the oldest metallacycles are the ferroles, which are dimetallacyclopentadiene complexes of the formula Fe(CR)(CO).  They are derived from coupling of alkynes as well as from the desulfurization of thiophenes.
The parent metallacyclobenzenes have the formula LM(CH). They can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex.