What is it about?

Coordination compounds of cobalt (III) [Co(NH3)5NO2]XY could undergo the isomerisation [Co(NH3)5NO2]XY -> [Co(NH3)5ONO]XY when irradiated by visible light. Photoisomerisation in such complexes lead to macroscopic mechanical effects, such as fragmentation of the crystal, bending or jumping. These phenomena were earlier called “photosalient effect” and “photomechanical effect”. Understanding its nature could help to develop molecular switches and other devices that could transform irradiation into mechanical work. It is well known that the rate of the photoisomerisation and photosalient effect in such complexes depend on the outer-sphere anions XY, because changing XY leads to changes in crystal structure.

Featured Image

Why is it important?

Free space around NO2 group and hydrogen bonds play key role in solid-state reactivity. Therefore, it is important to compare corresponding parameters in different structures to better understand how changing the outer-sphere anions XY might affect crystal structure and photoisomerisation. We have compared two isostructural compounds [Co(NH3)5NO2]ClNO3 and [Co(NH3)5NO2]BrNO3, their crystal structures and the changes arising during the photoisomerisation. Crystal structures of the reaction products [Co(NH3)5ONO]ClNO3 and [Co(NH3)5ONO]BrNO3 also were compared. Mainly we concentrated on free space in those structures and hydrogen bonds. We found that substituting Cl for a larger Br anion in [Co(NH3)5NO2]XNO3 leads to the expansion of crystal structure and free space. Therefore, mechanical response becomes less pronounced.


Relation between chemical composition, crystal structure and reactivity is one of the paradigms of the material sciences and solid state chemistry. It is a very fundamental and beautiful science, which always catches the attention. The possibility of explaining complex phenomena in simple terms and achieving a better understanding fascinates and inspires me. It was a great experience to be part of an excellent team and have an opportunity to work and consult with such experienced and interesting people.

Polina Kalinina
Novosibirsk State University

Read the Original

This page is a summary of: A comparison of the isostructural [Co(NH3)5NO2]XNO3 and [Co(NH3)5ONO]XNO3, X = Cl− or Br− in relation to nitro–nitrito linkage isome..., Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, May 2024, International Union of Crystallography,
DOI: 10.1107/s2052520624002816.
You can read the full text:



The following have contributed to this page