New RuII and OsII derivs. of the monovacant [alpha -PW11O39]7- anion ([PW11O39{M(DMSO)3(H2O)}]5- (M = Ru (1), Os (2)) and [PW11O39{Os(h6-p-cym)(H2O)}]5- (3)) have been synthesized and characterized. The binding mode of the d6-{MIIL3(H2O)}2+ moieties in these compds. is similar to that in the previously described [PW11O39{Ru(h6-p-cym)(H2O)}]5- (4) complex: bidentate, on two nonequivalent oxygen atoms of the lacuna, leading to a loss of the Cs symmetry of the parent anion, which thus plays the role of a prochiral bidentate ligand. The d. functional theory (DFT) (B3PW91) computation of the lowest unoccupied MOs of the {ML3(H2O)}2+ (M = Os, Ru; L3 = fac-(DMSO)3, h6-C6H6) fragments reveals the similarities between their electrophilic properties. The origin of the regioselectivity of the grafting was investigated through a DFT (B3PW91) anal. of (i) the HOMO 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.