The anion of the linear cyanobutadiynyl radical (C₅N^–) was detected by Cernicharo et al. as reported in 2008. The molecule was detected toward the carbon star CW Leonis using the IRAM 30m telescope as part of an extensive line survey carried out from 1995 to 2008. C₅N^– was found among a cluster of other unidentified lines, some of which were assigned to a vibratationally excited state of C₆H.

C₅N^– was the fifth gas phase anion to be identified in space, following C₆H^–, C₄H^– and C₈H^– in 2007 and C₃N^– in 2008. CN^– was subsequently discovered in 2010.