Synthesis and structural investigation of polyoxomolybdate coordination compounds displaying a tetranuclear core. Crystal and molecular structures of [Bu4N]2[Mo4O10(OMe)2(OC6H4O)2] and to the diazenido complexes [Bu4N]2[Mo4O6(OMe)2(HNC6H4O)2(NNC6H5)4] and [Bu4N]2[Mo4O6(OMe)2(C10H6O2)2(NNC6H5)4]. Comparison to a binuclear complex with the [Mo2(OMe)2(NNC6H5)4]2+ core, [Mo2(OMe)2(H2NC6H4O)]