In this paper we report the synthesis and the structural and magnetic properties of the series of ionic compounds with general formula: M-I[Co(bpy)(3)][Mo(CN)(8)] center dot nH(2)O (M-I= Li, n = 8 (1), M-I = K, n = 8 (2). M-I = Rb, n = 8 (3), M-I = Cs, n = 7.5 (4)). Solids 1-4 are characterized by the optical outer-sphere metal-to-metal charge transfer (MMCT) transition from Mo(IV) center to Co(III) center in the visible region and the (CoMoIV)-Mo-III reversible arrow (CoMoV)-Mo-II spin equilibrium strongly dominated by the (CoMoIV)-Mo-III form. We show a gentle thermal treatment of diamagnetic compounds 1-4 leading to the dehydrated forms 1a-4a, which reveal a significant increase of paramagnetic contribution (from 0.5 to 2% to 30-40%). The rehydration allows to recover the diamagnetic phases 1b-4b of compositions and properties similar to those of 1-4. The irradiation of the dehydrated form 2a within the MMCT band in the Superconducting Quantum Interference Device (SQUID) cavity at T = 10 K causes further increase of the (CoMoV)-Mo-II contribution giving the metastable phase annealed back to the 2a phase after heating above T = 290 K. The IR, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy, (XPS) spectroscopic data along with the magnetic data are interpreted in terms of strong modification of the (CoMoIV)-Mo-III reversible arrow (CoMoV)-Mo-II equilibrium occurring in these systems.