Thermal Decomposition Reactions of Analogous Penta-amminenitrosylcobalt(III) Complexes

The thermal behavior of five complexes, [Co(NH3)6]Cl3 (1), [Co(NH3)5NO2]Cl2 (2), [Co(NH3)5NO]Cl2 (3), [(NH3)5Co–N(O)–NO–Co(NH3)5]Cl4·2H2O (4), and [(NH3)5Co–NO–ON–Co(NH3)5]Cl4·4H2O (5), has been investigated. The pyrolysis-gaschromatographic technique has been successfully applied to the N2- and N2O analyses of the gases evolved on the pyrolysis of the complexes. The thermal decomposition of these complexes is characterized predominantly by the reduction process of Co(III) to Co(II). In the simple nitrosyl 3, the NO− group reduces the Co(III) ion and dissociates as NO. As for the remaining complexes, a thermally induced transient-state tends to dissociate H+ from the coordinated ammonia as follows : CoIII–NH3→[CoIII–NH2δ−···Hδ+]→CoII+NH2+H+. The NH2 species is subsequently decomposed as NH2→2/3NH3+1/6N2. Moreover, H+ is captured by an anionic species and/or the NH3 present in the system, that is, by Cl− and NH3 forming NH4Cl in the case of 1, by NO2− and NH3 forming NH4NO2, which then decomposes to N2 and H2O, in 2, and by N2O22− forming H2N2O2, which then decomposes to N2O and H2O, in 4. The 5 complex undergoes an exothermic decomposition, affording a larger amount of N2 and N2O than those to be expected from the above stoichiometry, suggesting an oxidative behavior of the bridged, dimeric (–NO–ON–)2− group towards ammonia.