A comparison of the isostructural [Co(NH 3 ) 5 NO 2 ]XNO 3 and [Co(NH 3 ) 5 ONO]XNO 3 , 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 ONO]BrNO 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 ONO]ClNO 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.