Dimolybdenum tetraacetate Mo2 (OAc)4 reacts with concentrated solutions of arsenic acid in air to form [Mo2(HAsO4)4]2- and some related ions which can be isolated with caesium or pyridinium cations: Cs2[Mo2(HAsO4)4]3 H2O (1), (C5NH6)3[Mo2(HAsO4)Cl] (2), (C5NH6)3 [Mo2(HAsO4)4Br] (3) and (C5NH6)2[Mo2(HAsO4)4]2H2O (4). The four compounds are stable indefinitely in air at room temperature. The last compound has been fully characterized structurally. It crystallizes in the space group P21/a with a = 11.057(2)Å, b = 10.720(2)Å, c = 10.303(4)Å, β = 91.56(3)°, V = 1220.8(6)Å3 and Z = 2. Interestingly, the overall structure can be described in terms of molybdenum-containing layers stacked in the c direction, in which arsenato ligands equatorially coordinating the Mo26+ unit play the role of an axial ligand for the next closest unit. The [Mo2(HAsO4)4]2- ion has a paddlewheel structure very similar to that of [Mo2(HPO4)4]2- and [Mo2(SO4)4]4-, 3-. The length of the Mo-Mo triple bond of (4) is 2.265(1)Å. Comparing the HPO42- and HAsO42- complexes, the electronic spectra have been found to be very similar, but the Raman spectra show a noticeable decrease of frequency from phosphato to arsenato-complexes for the Mo-Mo stretching modes. This decrease may come from a mechanical coupling between HAsO42- deformation modes and ν(Mo-Mo) and ν(Mo-O) vibrations, but is more likely due to a weakening of the Mo-Mo bond strained by the bridging ligand. © 1983 Verlag Chemie GmbH.
|Journal||Transition Metal Chemistry|
|Publication status||Published - 1 Aug 1983|