The role of the central atom X in the structure and reactivity of di-Ru-substituted gamma-Keggin polyoxometalates (POMS), gamma-[(Xn+O4)Ru-2(III)(OH)(2)(M-FM)(10)O-32]((8-n)-), where M-FM = Mo and W, and X = Al-III, Si-IV, P-V, and S-VI, was computationally investigated. It was shown that for both M-FM = Mo and W the nature of X is crucial in determining the lower lying electronic states of the polyoxoanions, which in turn likely significantly impacts their reactivity. For the electropositive X = Al-III the ground state is a low-spin state, while for the more electronegative X = S-VI the ground state is a high-spin state. In other words, the heteroatom X can be an '' internal switch '' for defining the ground electronic states of the gamma-M-2-Keggin POMs. The obtained trends, in general, are less pronounced for MFM = Mo than for W. On the basis of the comparison of the calculated energy gaps between low-spin and high-spin states of polytungstates and polymolybdates, we predict that the gamma-M-2-Keggin polytungstates could be more reactive than their polymolybdate analogues. For purposes of experimental verification the computationally predicted and evaluated polytungstate gamma-[(SiO4)Ru-2(III)(OH)(2)(-)(OH2)(2)W10O32](4-) was prepared and characterized.