To understand the initial hydration processes of MgCl2, we measured photoelectron spectra of MgCl2(H2O)n− (n = 0–6) and conducted ab initio calculations on MgCl2(H2O)n− and their neutral counterparts up to n = 7. A dramatic drop in the vertical detachment energy (VDE) was observed upon addition of the first water molecule to bare MgCl2−. This large variation in VDE can be associated with the charge-transfer-to-solvent (CTTS) effect occurring in the MgCl2(H2O)n− clusters, as hydration induces transfer of the excess electron of MgCl2− to the water molecules. Investigation of the separation of Cl−–Mg2+ ion pair shows that, in MgCl2(H2O)n− anions, breaking of the first Mg–Cl bond occurs at n = 4, while breaking of the second Mg–Cl bond takes place at n = 6. For neutral MgCl2(H2O)n clusters, breaking of the first Mg–Cl bond starts at n = 7.
