  We report on the first detection of C3N- and C5N- towards the cold dark core
TMC-1 in the Taurus region, using the Yebes 40 m telescope. The observed
C3N/C3N- and C5N/C5N- abundance ratios are 140 and 2, respectively; that is
similar to those found in the circumstellar envelope of the carbon-rich star
IRC+10216. Although the formation mechanisms for the neutrals are different in
interstellar (ion-neutral reactions) and circumstellar clouds
(photodissociation and radical-neutral reactions), the similarity of the
C3N/C3N- and C5N/C5N- abundance ratios strongly suggests a common chemical path
for the formation of these anions in interstellar and circumstellar clouds. We
discuss the role of radiative electronic attachment, reactions between N atoms
and carbon chain anions Cn-, and that of H- reactions with HC3N and HC5N as
possible routes to form CnN-. The detection of C5N- in TMC-1 gives strong
support for assigning to this anion the lines found in IRC+10216, as it
excludes the possibility of a metal-bearing species, or a vibrationally excited
state. New sets of rotational parameters have been derived from the observed
frequencies in TMC-1 and IRC+10216 for C5N- and the neutral radical C5N.
