NITRITO-NITRO LINKAGE ISOMERIZATION IN THE SOLID-STATE .2. COMPARATIVE-STUDY OF THE STRUCTURES OF NITRITO-PENTAAMMINECOBALT(III) DICHLORIDE AND NITROPENTAAMMINECOBALT(III) DICHLORIDE

The mechanism of the isomerization in the solid state of nitritopentaamminecobalt(III) dichloride to the thermodynamically more stable nitro compound and of the reverse photochemical nitro → nitrito reaction have been investigated. Freshly synthesized crystals of [Co(NH3)5ONO]Cl2 and crystals of the same compound aged for 1 half-life have been used for structure determinations. The intensity data were collected by a computer-controlled diffractometer. The two structures are orthorhombic, space group P21nb, with Z = 4. Monomeric cobalt complexes and chloride ions are linked by electrostatic forces and by a network of hydrogen bonds. The coordination around cobalt is close to octahedral. A comparison of the X-ray structures of the nitrito and nitro compounds and the changes in the powder diffractograms with time indicate that the thermal nitrito → nitro isomerization consists of two consecutive steps. The first step involves an intramolecular change of the coordination mode, most likely via a seven-coordinated transition state. The second step consists of a slow rearrangement which can be interpreted as a cooperative ~90° rotation of half of the number of the coordination polyhedra, or as a pseudo-C3-rotation or an intramolecular twist. The nitro → nitrito photoisomerization gives a nitrito compound with a structure different from the one originally synthesized. The photoreaction is probably also intramolecular, proceeding via a seven-coordinated transition state. © 1979, American Chemical Society. All rights reserved.