ar b ony l ▁ ▁( C 4 H 4 ) Fe ( CO ) 3 , ▁where ▁C 4 H 4 ▁is ▁the ▁otherwise ▁un stable ▁cy c lob ut ad iene . ▁ ▁Rece iving ▁the ▁greatest ▁attention ▁are ▁complex es ▁of ▁the ▁cy clo hex ad ien es , ▁the ▁parent ▁organ ic ▁ 1 , 4 - d ien es ▁being ▁available ▁through ▁the ▁Bir ch ▁redu ctions . ▁▁ 1 , 4 - D ien es ▁is omer ize ▁to ▁the ▁ 1 , 3 - d ien es ▁upon ▁complex ation . ▁ ▁Fe ( CO ) 5 ▁react s ▁in ▁d icy c lop ent ad iene ▁to ▁form ▁[ Fe ( C 5 H 5 )( CO ) 2 ] 2 , ▁cy c lop ent ad ien yl ir on ▁dic ar b ony l ▁dim er . ▁ ▁This ▁comp ound , ▁called ▁" F p ▁dim er " ▁can ▁be ▁considered ▁a ▁hy brid ▁of ▁fer roc ene ▁and ▁Fe ( CO ) 5 , ▁although ▁in ▁terms ▁of ▁its ▁react ivity , ▁it ▁res emb les ▁neither . ▁ ▁Other ▁uses ▁In ▁Europe , ▁iron ▁pent ac ar b ony l ▁was ▁once ▁used ▁as ▁an ▁anti - k no ck ▁agent ▁in ▁pet rol ▁in ▁place ▁of ▁t etra eth yl le ad . ▁ ▁Two ▁more ▁modern ▁alternative ▁fuel ▁add it ives ▁are ▁fer roc ene ▁and ▁m eth yl cyc lop ent ad ien yl ▁mang an ese