Cp for short, this pentagonal molecule is useful and interesting because it is aromatic when charged. That is, the Cp- (cyclopentadienyl) ion is symmetric (it is C5H5 and flat) so it can readily form metal-pi interactions. One of the first zero valent metal complexes (zinc, i thinc) was with two Cp ions sanwiching the metal between them. It has since become much used to complex metals or as a production route for cyclophanes or other exotic structures.