The nitrito to nitro spontaneous isomerization reaction of the complexes [Co(MeNH 2 ) 5 (ONO)] 2 + , cis-[Co(MeNH 2 )-
trans-[CoL 14 (ONO)] 2 + and cis-[CoL 14 (ONO)] 2 + has been studied as a function of temperature and pressure. The corresponding base-catalyzed reactions of the systems with monodentate ligands have also been studied as a function of temperature and pressure. The results produce thermal and baric activation parameters that can be as a whole related to an ordered expansion to go to the transition state. This is particularly true for the (N) 5 macrocyclic L 14 systems (L 14 = 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) where the importance of hydrogen bonding becomes evident as for related studies. For systems with a high trans influence the results indicate a clear increase of the degree of dissociation to go to the transition state, very much in line with what has been observed for previously studied substitution reactions. Thus, the results agree with the fact that the nitrito to nitro isomerization reactions, in spite of being of intramolecular nature as established by very clear results, follow a reactivity pattern very similar to that observed for the substitution reactions of neutral ligands on the same complexes. That is, the transition state of the process has a high resemblance to that of the dissociatively activated substitution reactions of the Co(III) complexes.