In the present work, absolute hydration enthalpies are calculated from ion absolute hardness for a series of +1 and -1 ions. The calculated values are compared with those previously reported (Housecroft, 2017)  and relationships between V-m(-1/3) and absolute hardness are stablished. The following empirical equations have been derived, for cations and anions, respectively: Delta H-hyd degrees = -(9.645 eta(+) + 245.930) and Delta H-hyd degrees = -(64.601 eta(-) + 12.321). In such equations, eta(+) and eta(-) are the absolute hardness. It is shown that for d block monocations (Cu+, Ag+ and Au+), hydration enthalpy is closely related with Clementi effective nuclear charge by the equation: Delta H-hyd degrees = -(9.645 eta(+) + 245.930) (Z(eff)/(n - 1)), where n is the main quantum number. Furthermore, is shown that a typical VBT parameter (V-m(-1/3)) is related with eta(+) and eta(-) values and so, with the energies of the frontier orbitals, that is, is stablished a direct relationship between a structural parameter available by X-ray data and the energy of atomic/molecular orbitals. (C) 2017 Elsevier B.V. All rights reserved.