We have carried out a coincidence experiment between energy-resolved resonant Auger electrons and mass-resolved ions on ${\mathrm{CH}}_{3}\mathrm{F}$ molecules following F $1s$ and C $1s$ excitation to the lowest unoccupied C--F antibonding molecular orbital ${\ensuremath{\sigma}}_{\mathrm{CF}}^{*}$. We found a strong enhancement of the ${\mathrm{F}}^{+}$ or ${\mathrm{CH}}^{+}$ ion production in coincidence with the F $KVV$ and C $KVV$ spectator Auger electrons, respectively, in the wide binding energy range of 28--36 eV. This site-selective ion production is interpreted as a consequence of the resonant Auger emission taking place in the transient region where the C--F elongation caused by the core excitation transforms the molecular valence orbitals gradually into nonoverlapping valence orbitals of each fragment.