Synthesis, structure and properties of molybdenum(VI) oxalate complexes of the types M2[Mo2O5(C2O4)2(H2O)2] and M2[MoO3(C2O4)] (M=Na, K, Rb, Cs)

The reaction of molybdenum(VI) oxide with oxalic acid or alkali oxalate and alkali halides results in the formation of two series of molybdenum(VI) oxalate complexes: one of the general formula M2[Mo2O5(C2O4)2(H2O)2] containing the Mo2O5 core and the other of the formula M2[MoO3(C2O4)] with a MoO3 core (M=Na, K, Rb, Cs). Both series were characterized by chemical analysis, ESR, UV and IR spectroscopy, thermogravimetry, differential scanning calorimetry, X-ray powder method and some of them by single-crystal X-ray structure analysis. Complexes of the first series adopt dimeric structures, the second series infinite polymeric structures. In both types of structures molybdenum ions are six-coordinated being surrounded by terminal oxo-oxygens, bridging oxygens and bidentate bonded oxalate ligands. When exposed to UV light all these complexes in the solid state exhibit photochromic behavior changing color from colorless to green–brown. These changes are remarkably more pronounced in the complexes with dimeric structures. There is definite correlation between their coloration and the UV induced ESR signal indicative for molybdenum(V). Such behavior is explained by the partial reduction of Mo(VI) to Mo(V) only at the crystal surfaces. This is also the explanation why K2[Mo2O5(C2O4)2(H2O)2] was so far described as being red or pale reddish tan. All attempts to prepare the corresponding lithium complexes were unsuccessful. They are most probably very unstable because of the small radius of the lithium ion.