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A series of oxomolybdate(V) complexes with methanol was prepared by the reaction of (PyH) 5 (3) or as an orthorhombic polymorph (4). After prolonged reaction times further assembly of dinuclear fragments takes place and tetranuclear

6− revealed the isomer with the staggered conformation of the µ 4 -oxalate ion to be of the lowest energy.

Reactions of mononuclear oxohalomolybdates() with squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione) afforded two types of {Mo 2 O 4 } 2ϩ containing species with the squarato ligand engaged in a µ 2 -1,2-bis(monodentate) binding mode. In the presence of pyridine (Py) or 3,5-lutidine (3, 5-Lut) 1c were obtained. A pair of nitrogen donor ligands and a squarate oxygen occupy the three coordination sites of each metal in the {Mo 2 O 4 } 2ϩ unit in 1a, 1b and 1c. The squarate functions as a third, auxiliary bridge between a pair of molybdenum atoms at an average distance of 2.55 Å. Using 2,4,6-collidine (Col) afforded (ColMe) 4 [Mo 4 O 8 (C 4 O 4 ) 4 ]ؒ2MeOHؒ2Col 2a (ColMe ϩ = N-methylcollidinium cation, NC 9 H 14 ϩ ) and (ColH) 4 (PyEt)[Mo 4 O 8 (C 4 O 4 ) 4 ]Br 2b (ColH ϩ = protonated 2,4,6-collidine, NC 8 H 12 ϩ ; PyEt ϩ = N-ethylpyridinium cation, NC 7 H 10 ϩ ). The tetranuclear anions of 2a and 2b consist of a rare, cube-like {Mo 4 O 4 (µ 3 -O) 4 } 4ϩ core with four squarato ligands attached to its periphery. The squarato ligands in the [Mo 4 O 8 (C 4 O 4 ) 4 ] 4Ϫ anions bridge pairs of molybdenum atoms from neighbouring {Mo 2 O 4 } 2ϩ units, spanning an average distance of 3.43 Å. The compounds were fully characterized by infrared vibrational spectroscopy, NMR spectroscopy, elemental analyses and X-ray diffraction studies.