What is "pentacyanocyclopentadiene"

Pentacyanocyclopentadiene is a derivative of cyclopentadiene with five cyano groups with the molecular formula CH(CN). The corresponding anion, pentacyanocyclopentadienide, is a ligand with the molecular formula C(CN). In contrast to other anions based on a C ring unit it binds to metals through the pendant cyano groups rather than the C ring. The anion was first synthesised by Webster in the 1960s and its conjugate acid much later on. More recently Wright has discovered its extensive coordination chemistry.