Reactivity of different isomers of N2F2
According to Dinitrogen Difluoride Chemistry Inorg. Chem., 2010, 49 (15), pp 6823–6833:  


  This increased reactivity of the cis-isomer can be readily
  explained. In both $\ce{N2F2}$ isomers, each nitrogen atom
  possesses a sterically active free valence electron pair.
  The transformation of $\ce{N2F2}$ to $\ce{N2F+}$ requires the conversion
  of the $\ce{N=N}$ double bond into a $\ce{N#N}$ triple bond
  by an overlap of electrons from these two free valence electron pairs. Obviously, this overlap is greatly facilitated
  in cis-$\ce{N2F2}$, where these free pairs are on the same
  side of the molecule.