12 pages ; International audience ; Reactions of pyrazine (Pyz = C4H4N2) with [MoOCl4(H2O)]– or [MoOBr4]– afforded a series of complexes, dinuclear [(MoOX4)2(Pyz)]2– (X = Cl, Br), [{MoOCl3(Pyz)}2O]2– and tetranuclear [(Mo2O4Cl4)2(Pyz)2]4– anions, with N-donor ligands engaged either in monodentate or bidentate bridging coordination. A dinuclear [(MoOX4)2(Pyz)]2– ion, which may be viewed as a linkage of two mononuclear subunits with a bidentate pyrazine, crystallizes either as a PyH+ (pyridinium cation, C5H5NH+; compound 1), a {(C2H5)4N}+ (compound 2) or a {(C6H5)4P}+ salt (compounds 3 and 4). In (PyH)2- [{MoOCl3(Pyz)}2O] (5) with an anti-{Mo2O3}4+ core, pyrazine is bound to the metal through only one nitrogen atom. The tetranuclear [(Mo2O4Cl4)2(Pyz)2]4– ion, a linkage of two metal–metal bonded {Mo2O4}2+ cores with a pair of bidentate pyrazines, is found in two compounds, (MeNC5H5)2(PyzH2)- [(Mo2O4Cl4)2(Pyz)2] (6) [MeNC5H5 + = N-methylpyridinium cation, PyzH2 2+ = pyrazinium(2+) cation] and (PyH)4- [(Mo2O4Cl4)2(Pyz)2]·2CH3CN (7). In the presence of methanol, dinuclear {(C6H5)4P}2[(MoOBr4)2(Pyz)]·2CH3CN (4) was transformed directly into tetranuclear (PyH)4[(Mo2O4Br4)2- (Pyz)2]·2CH3CN (8). All compounds were fully characterized by X-ray crystallography and IR spectroscopy. The compounds containing the [(MoOX4)2(Pyz)]2– ions display an irreversible electrochemical reduction behaviour. Temperature- dependent magnetic measurements on {(C6H5)4P}2- [(MoOX4)2(Pyz)]·2CH3CN (X = Cl for 3, X = Br for 4) reveal a weak ferromagnetic coupling across the pyrazine bridge between a pair ofMoV S = 1/2 spins. The magnetic properties of {(C2H5)4N}2[(MoOCl4)2(Pyz)] (2), another compound with dinuclear complex anions, are dominated by antiferromagnetic intermolecular interactions.