We have carried out a study, by ab initio methods, of the solvent effect on [(NH3)(5)Ru-pyrazine](m+) (m=2 and 3) complexes, which are of interest as basic units of metallic chains with potential application in molecular electronics and photonics. We have performed multireference configuration interaction (CI) calculations in which solvent effects are included by the Polarizable Continuum Model. A new method for the estimate of the size of the cavities in this approach, which has proven to be suitable for the specific case under study, is also proposed. Our calculations account for the red shift of the metal-to-ligand charge transfer band, observed experimentally for the Ru(LI) compound as the solvent donor number increases, and furnish an explanation with a solid theoretical foundation. For the Ru(III) compound we find that the ground state configuration is different in vacuum and in electron donor solvents.