Covalent hydrates, oxidation stabilization

The maximum stability for vanadium as the V3 4 ion in acidic solution can be understood in terms of the maximization of the enthalpy of hydration for the +3 ion, above which hydrolysis alters the form and stability of the higher states. The + 4 and + 5 states are considerably electronegative compared to the lower oxidation states, and are able to engage in covalent bonding to ligand oxide ions to form V=0 bonds. 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

---