The protonation of a hydroxide anion (OH-) located between two magnesium cations (Mg2+) in aqueous solution has been investigated by first-principles metadynamics simulation. We observe that the complex Mg2+-OH--Mg2+ is stabilized by the coparticipation of the hydroxide anion to the first hydration shells of both the Mg2+ cations. Contrary to the cases of OH- in pure water, the transfer of protons in the presence of the divalent metal ions turns out to be a slow chemical event. This can be ascribed to the decreased proton affinity of the bridging OH-. Metadynamics simulation, used to overcome the difficulty of the long time scale required by the protonation of the bridging OH-, has shown that the system possesses a great stability on the reactant state, characterized by a bioctahedral (6,6) solvation structure around the two Mg2+ cations. The exploration of the free energy landscape shows that this stable bioctahedral configuration converts into a lower coordinated (5,6) structure, leading to a proton transfer from a water molecule belonging to the first solvation shell of the Mg2+ ion having the lower coordination to the bridging OH-; the free energy barrier for the protonation reaction is 11 kcal/mol, meaning that the bridging hydroxide is a weak base. During the proton transfer, the bridging OH-reverts to an H2O molecule, and this breaks the electrostatic coupling of the two Mg2+ ions, which depart independently with their own hydration shells, one of which is entirely formed by water molecules. The second one carries the newly created OH-. Our results show that the flexibility in the metal coordination plays a crucial role in both the protonation process of the bridging OH- and the separation of the metal cations, providing useful insight into the nature of proton transfer in binuclear divalent metal ions, with several biological implications, such as, for instance, transesterification of catalytic RNA.