Thermoanalytical study of linkage isomerism in coordination compounds. Part III: A DSC study on the effect of counterion on the solid state isomerization of nitro and nitrito linkage isomers of pentaamminecobalt(III) complexes

• The effect of counterion on solid state thermal isomerization of nitro and nitrito linkage isomers of pentaamminecobalt(III) complexes studied by DSC.
• The different prepared salts of both isomers are in metastable state at room temperature which convert to an equilibrium stale state upon heating.
• Change of the counterion affects thermodynamic parameters of the isomerization reaction, but the nitro isomer is always more stable than the nitrito one.
• The kinetic parameters of the linkage isomerization depend significantly on the nature of the counterion.

Solid state thermal interconversion of [Co(NH3)5NO2]X2 (nitro isomer) and [Co(NH3)5ONO]X2 (nitrito isomer) complexes (X = Cl−, Br−, I−, PF6−) was studied by DSC at several heating rates. The isomerization of pure sample of both isomers leads to an equilibrium stable state in which nitro and nitrito isomers are major and minor components, respectively. Change of the counterion affects thermodynamic parameters of the isomerization reaction, but the nitro isomer is always more stable than the nitrito one. The equilibrium constant (nitro to nitrito molar ratio) increases in the series Cl− ≈ Br− < PF6− < I−. The kinetic parameters of isomerization of both isomers were determined by Kissinger method. It was founded that the rate of interconversion of different salts of both isomers decreases in the series Cl− > Br− > PF6− > I−. The negative activation entropy supports an associative mechanism with a seven coordinate transition state in solid state.

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