Cyclononatetraene
Cyclononatetraene is an organic compound with the formula C9H10. It was first prepared in 1969 by protonation of the corresponding aromatic anion. It is unstable and isomerizes with a half-life of 50 minutes at room temperature to 3a,7a-dihydro-1H-indene via a thermal 6π disrotatory electrocyclic ring closing. Upon exposure to ultraviolet light, it undergoes a photochemical 8π electrocyclic ring closing to give bicyclonona-2,4,6-triene.
Cyclononatetraenyl anion
Cyclononatetraenyl anion is a 10π aromatic system. Two isomers of the cyclononatetraenyl anion are known: the trans,''cis,cis,cis isomer and the all-cis isomer. The former is less stable and isomerizes to the latter upon warming from –40 °C to room temperature.The all-cis isomer of C9H9− can be prepared by treatment of 9-chlorobicyclonona-2,4,6-triene with lithium or potassium metal. Despite the ring strain resulting from having C–C–C bond angles of 140° instead of the ideal 120° for sp2 carbon, this species is believed to be planar and to possess D''9h symmetry. The lithium salt was found to react with cyclopentadiene to give lithium cyclopentadienide, showing that cyclononatetraene is a weaker acid than cyclopentadiene.