New Ru-II and Os-II derivatives of the monovacant [alpha-PW11O39](7-) anion ([PW11O39{M(DMSO)(3)(H2O)}](5-) (M = Ru (1), Os (2)) and [PW11O39{Os(eta(6)-p-cym)(H2O)}](5-) (3)) have been synthesized and characterized. The binding mode of the d(6)-{(ML3)-L-II(H2O)}(2+) moieties in these compounds is similar to that in the previously described [PW11O39{Ru(eta(6)-p-cym)(H2O)}](5-) (4) complex: bidentate, on two nonequivalent oxygen atoms of the lacuna, leading to a loss of the C-s symmetry of the parent anion, which thus plays the role of a prochiral bidentate ligand. The density functional theory (DFT) (B3PW91) computation of the lowest unoccupied molecular orbitals of the {ML3(H2O)}(2+) (M) Os, Ru; L-3 = fac-(DMSO)(3), eta(6)-C6H6) fragments reveals the similarities between their electrophilic properties. The origin of the regioselectivity of the grafting was investigated through a DFT (B3PW91) analysis of (i) the highest occupied molecular orbital of [alpha-PW11O39] (7-) and (ii) the relative energies of the different potential regioisomers obtained by a bidentate grafting of the {ML3(H2O)}(2+) moiety onto the lacuna of [alpha-PW11O39](7-). The role of the water ligand in the stabilization of this peculiar structure was studied.