The stability of particular oxidation state of a metal in aqueous solution is determined by

The stability of a particular oxidation state of a metal in aqueous solution  determined Thermodynamically  

Step−1 enthalpy of sublimation of the metal

The enthalpy of sublimation, or heat of sublimation, is the heat required to change one mole of a substance from solid-state to a gaseous state at a given combination of temperature and pressure, usually standard temperature and pressure.

M_metal​(S)→M_metal​(g)

Step−2 it will go to +1 oxidation state where ionization energy is required for it.

Step−3Then we put it in an aqueous solution where enthalpy of hydration of the metal ion occurs.