The nickel(II) and copper(II) complexes of a new scorpionand in which the side chain has been functionalized with the powerful chromophore 4-amino-7-nitro-benzo-2-oxa-1,3-diazole have been prepared. The pendant arm of the new ligand undergoes pHdependent complexation/decomplexation to the metal centre, with simultaneous deprotonation of the aniline nitrogen atom of the pendant arm, thus inducing drastic modifications of the spectral features of the chromophore and a sharp colour change. The pKa values observed for both complexes change according to the metal promoting the deprotonation, being lower for the Ni complex than for the Cu species, because of the different coordination geometries adopted by the two ions.