A comparison of the isostructural Co(NH3)5NO2XNO3 and Co(NH3)5ONOXNO3, X = Cl− or Br− in relation to nitro–nitrito linkage isomerization and photomechanical effects

A new photoactive cobalt coordination compound, Co(NH 3 ) 5 NO 2 BrNO 3 ( I ), was obtained. Its crystal structure was shown to be isostructural with previously known Co(NH 3 ) 5 NO 2 ClNO 3 ( II ) for which linkage isomerization accompanied with mechanical response of the crystal has been already reported. Single crystals of I are transformed into nitrito isomer Co(NH 3 ) 5 ONOBrNO 3 ( III ) on irradiation with blue light (λ = 465 nm) without being destroyed. The crystal structure of III was also solved using single-crystal X-ray diffraction and compared with previously known Co(NH 3 ) 5 ONOClNO 3 ( IV ). A detailed comparison of the structures of I , II , III and IV , including unit-cell parameters, the distribution of free space (in particular, reaction cavities around the nitro ligand), the lengths of hydrogen bonds, coordination and Voronoi–Dirichlet polyhedra has been performed. Single-crystal X-ray diffraction data were complemented with IR spectra. The effect of the replacement of Cl − by Br − on the crystal structure and on the nitro–nitrito photoisomerization is discussed.