The decawolframate anion reduced by two electrons, W10O326-, is diamagnetic, and its two "extra" electrons delocalize mainly among its eight equatorial wolfram sites. In this work, we combine a phenomenological Hamiltonian with first-principles calculations to explain the origin of these properties. Through ab initio calculations and effective Hamiltonians on fragments, we determine the values of the magnetic exchange parameters, J, the transfer integrals, t, the Coulombic repulsions, V, and the orbital energies, c. Then, by introducing these parameters in a model Hamiltonian simulating the whole molecule, one obtains that the singlet-triplet gap is 780 meV and that more than a 90% of the "extra" electron density resides on the eight equatorial wolfram ions. An analysis of the interplay between these parameters indicates that electron-transfer processes play a dominant role while magnetic exchange has only a minor influence.