A series of molybdenum(v) complexes with the oxalato ligand engaged in different binding roles -: An unusual staggered conformation of the μ4-oxalate in [{Mo2O4(η2-C2O4)2}2(μ4-C2O4)]6-

Solvothermal. reactions between mononuclear halooxo-molybdates(v) and oxalic acid afforded smaller oligonuclear complexes with the oxalate ion engaged in different binding modes. The oxalate ion coordinates as a bidentate terminal ligand to each metal centre of the {Mo2O4}(2+) unit to form the anion of (MeNC5H5)(2)[Mo2O4(eta(2) -C2O4)(2)py(2)] (1; py = pyr-idine; MeNC5H5+ = N-methylpyridinium cation). (MeNC5H5)(6)[{Mo2O4 (eta(2) -C2O4)(2)}(2)(mu(4)-C2O4)] (2) with a tetra-dentate oxalate ion connecting two dinuclear subunits is a very rare example of a coordination compound with the oxalato ligand in a completely staggered conformation: the two CO2 entities of the mu(4)-oxalate ion are rotated by 90degrees with respect to each other. Ab initio MO calculations on [{Mo2O4(eta(2)- C2O4)(2)}(2)(mu(4)-C2O4)](6-) revealed the isomer with the staggered conformation of the mu(4)-oxalate ion to be of the lowest energy. The cyclic octanuclear anion of (MeNC5H5)(2)[Mo8O16-(OMe)(8)(mu(8)-C2O4)] (3) consists of four dinuclear subunits arranged around the central oxalate ion and additionally linked by pairs of methoxide bridges. Each oxygen atom of the oxalate ion bridges a pair of metal centres not connected by a metal-metal bond. The anionic part of [MeNC5H2-(Me)(3)](2)[HNC5H2(Me)(3)](2)[Mo2O3(eta(2) -C2O4)(4)] (4) [MeNC5H2-(Me)(3)(+) = N-methylcolhdinium cation; HNC5H2(Me)(3)(+) = pro-tonated 2,4,6-collidine] possesses an anti-{Mo2O3}(4+) structural fragment with a pair of bidentate oxalate ions coordinated to each molybdenum atom. The compounds were fully characterized by X-ray structural analysis and infrared spectroscopy. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).