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]·<i>n</i>H₂O (M^I = Li, <i>n</i> = 8 (<b>1</b>), M^I = K, <i>n</i> = 8 (<b>2</b>), M^I = Rb, <i>n</i> = 8 (<b>3</b>), M^I = Cs, <i>n</i> = 7.5 (<b>4</b>)). Solids <b>1</b>−<b>4</b> 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 Co^IIIMo^IV Co^IIMo^V spin equilibrium strongly dominated by the Co^IIIMo^IV form. We show a gentle thermal treatment of diamagnetic compounds <b>1</b>−<b>4</b> leading to the dehydrated forms <b>1a</b>−<b>4a</b>, which reveal a significant increase of paramagnetic contribution (from 0.5 to 2% to 30−40%). The rehydration allows to recover the diamagnetic phases <b>1b</b>−<b>4b</b> of compositions and properties similar to those of <b>1</b>−<b>4</b>. The irradiation of the dehydrated form <b>2a</b> within the MMCT band in the Superconducting Quantum Interference Device (SQUID) cavity at <i>T</i> = 10 K causes further increase of the Co^IIMo^V contribution giving the metastable phase annealed back to the <b>2a</b> phase after heating above <i>T</i> = 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 Co^IIIMo^IV Co^IIMo^V equilibrium occurring in these systems.< Réduire