Synthesis and chemical characterization of complexes containing semiquinone units bridged by pentaoxodimolybdate groups. Structural and electrochemical studies of [(n-C4H9)4N]2[Mo4O10(C6H2O4)2] and [(n-C4H9)4N]3[Mo6O15(C6O6)2] and of an analogous tetraoxydibenzofuran complex, [(n-C4H9)4N]2[Mo4O10(C12H4O5)2], prepared from metal-mediated radical coupling of 1,2,4-trihydroxybenzene precursors