The whole series of Lindqvist-type nitrosyl derivatives (n-BU4N)(3)[M(5)O(18){M’(NO)}] (M, M’ = Mo. W) has been synthesized through the reaction of [Mo(NO){MeC(NH2)NO} (acac)(2)] or [W(NO)Cl-3(MeCN)(2)] with (n-Bu(4)N)(2-)[Mo2O7] or (n-Bu(4)N)(2)[WO4] in MeCN. The electronic features of the [M(5)O(18){M’(NO)}](3-) anions have treen investigated by vibrational, electronic and multinuclear (N-14, O-17, Mo-95, and W-183) magnetic resonance spectroscopy, and by electrochemistry. A full assignment of the NMR spectra has been achieved. Chemical shifts are dominated by the paramagnetic term of the shielding. O-17 NMR data and electrochemical reduction data show that the expected effect of the whole charge increase with respect to the parent oxoanions [M(6)O(19)](2-) is partly offset by the electron-withdrawing effect of the nitrosyl ligand. All four species are essentially localized mixed-valence; complexes, with five d(0)-M(VI) centers and one d(4)-M’(II) center; however, some electronic delocalization is supported by Mo-95 and W-183 NMR data and by electrochemical data.