In this paper we present a theoretical study of the linkage isomerization in [Co(NH3)(5)ONO](2+). In particular, we have used for our analysis a hybrid density functional/Hartree-Fock method for the electronic computations (the so-called B3LYP), while a recent version of the polarizable continuum model has been considered to model solute-solvent interactions. A particular effort has been devoted to understand the kinetics of the reaction and to assess the nature of the transition state. Our results show that this reaction is only marginally exothermic in the gas phase, while solvent significantly shifts the equilibrium toward the nitro isomer. We have also identified the transition state of the reaction, which can be described as a heptacoordinate structure with the NO2 group in eta (2) coordination.