Synthesis and structural investigation of polyoxomolybdate coordination compounds displaying a tetranuclear core. Crystal and molecular structures of Bu4N2Mo4O10(OMe)2(OC6H4O)2 and to the diazenido complexes Bu4N2Mo4O6(OMe)2(HNC6H4O)2(NNC6H5)4 and Bu4N2Mo4O6(OMe)2(C10H6O2)2(NNC6H5)4. Comparison to a binuclear complex with the Mo2(OMe)2(NNC6H5)42+ core, Mo2(OMe)2(H2NC6H4O)