In organometallic chemistry, the pentadienyl anion is a ligand, the acyclic analogue of the more common cyclopentadienyl. It is generated by deprotonation of pentadiene. A number of complexes are known, including the analogue of ferrocene, Fe(C5H7)2. Only few pentadienyl complexes feature C5H7 ligands. More common is the dimethyl analogue 2,4-Me2C5H5. Additionally, many pentadienyl ligands are cyclic, being derived from the addition of hydride to η6-arene complexes or hydride abstraction from cyclohexadiene complexes.
The first pentadienyl complex to be reported was derived from protonolysis of a complex of pentadienol:
- Fe(C5H7OH)(CO)3 + H+ → [Fe(C5H7)(CO)3]+ + H2O
Treatment of this cation with sodium borohydride gives the pentadiene complex:
- [Fe(C5H7)(CO)3]+ + H− → Fe(C5H8)(CO)3
In organic chemistry, the pentadienyl radical, C
7, is of some significance as an especially stabilized radical. The radical is delocalized over five carbon centers. Consequently the C−H bond in the diene (CH2(CH=CH2)) is especially weak. Fats derivatives containing this "doubly allylic" group are collectively called drying oils. They tend to polymerize in a useful way upon exposure to air.
- Lothar Stahl, Richard D. Ernst (2007). "Pentadienyl Complexes of the Group 4 Transition Metals". Advances in Organometallic Chemistry. 55: 137–199. doi:10.1016/S0065-3055(07)55003-3.
- Richard D. Ernst (1988). "Structural and reactivity patterns in transition-metal-pentadienyl chemistry". Chem. Rev. 88: =1255–1291. doi:10.1021/cr00089a013.
- Mahler, J. E.; Pettit, R. (1962). "Pentadienyl and Hexadienyl Carbonium Ions as Ligands in Stable Complex Cations". J. Am. Chem. Soc. 84: 1511–2. doi:10.1021/ja00867a051.