Synthesis, structure and properties of molybdenum(VI) oxalate complexes ofthe types M-2[Mo2O5(C2O4)(2)(H2O)(2)] and M-2[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) ox alate complexes: one of the general formula M-2[Mo2O5(C2O4)(2)(H2O)(2)] con taining the Mo2O5 core and the other of the formula M-2[MoO3(C2O4)] with a MoO3 core (M = Na, K, Rb, Cs). Both series were characterized by chemical a nalysis, ESR, UV and IR spectroscopy, thermogravimetry, differential scanni ng calorimetry, X-ray powder method and some of them by single-crystal X-ra y structure analysis. Complexes of the first series adopt dimeric structure s, the second series infinite polymeric structures. In both types of struct ures molybdenum ions are six-coordinated being surrounded by terminal ore-o xygens, bridging oxygens and bidentate bonded oxalate ligands. When exposed to UV light all these complexes in the solid state exhibit photochromic be havior changing color from colorless to green-brown. These changes are rema rkably more pronounced in the complexes with dimeric structures. There is d efinite correlation between their coloration and the UV induced ESR signal indicative for molybdenum(V). Such behavior is explained by the partial red uction of Mo(VI) to Mo(V) only at the crystal surfaces. This is also the ex planation why K-2[Mo2O5(C2O4)(2)(H2O)(2)] was so far described as being red or pale reddish tan. All attempts to prepare the corresponding lithium com plexes were unsuccessful. They are most probably very unstable because of t he small radius of the lithium ion. (C) 2000 Elsevier Science S.A. All righ ts reserved.