The reactivity of trans- and cis-phenyldiazene (H-N=N-C6H5) induced by the internal rotation of the phenyl group is studied through the characterization of the profiles of the energy, the chemical potential, the hardness, and the local electronic properties. Phenyldiazene presents two stable isomers when the phenyl group is trans or cis with respect to the hydrogen atom bonded to the second nitrogen. The trans isomer is planar, whereas in the cis isomer, the phenyl group is tilted by about 40degrees with respect to the molecular plane. The change of local reactivity indexes induced by the low-energy internal rotation motion explains the selectivity trend observed when cis-phenyldiazene is coordinated by a metallic complex.