Reactivity of different isomers of N2F2 $\ce{ N2F2}$ exists in two forms cis and trans. Today my chemistry teacher tells me that among the two only cis form takes part in reactions and that it is more reactive. But I have read elsewhere that trans is less stable. As far as I understand less stable compounds are usually more reactive. So why this discrepancy?
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.