Crystal and molecular structure of molybdenum tetrachloride oxide by neutron and X-ray diffraction

Abstract

The crystal and molecular structure of MoOCl₄ has been determined by neutron and X-ray diffraction at 293 K and 77 K. A model, derived from consideration of the cubic close-packing together with an X-ray Patterson synthesis with Weissenberg data, was refined by the neutron powder profile-fitting technique to R=Σ|y_o–Y_c|/Σy_o= 0.094 (293 K) and 0.081 (77 K). The compound MoOCl₄ is triclinic, space group P, with unit-cell dimensions a= 6.255(4), b= 7.236(3), c= 7.134(3)Å, α= 103.49(3), β= 107.02(6), and γ= 92.82(4)° at 293 K. There are no large changes in the structure on cooling to 77 K. The Mo atoms occur in adjacent octahedral holes (Mo–Mo = 4.12 Å) but there is no dimer formation as in MoCl₅. The configuration around Mo is a square pyramid, with four basal chlorine atoms and an apical oxygen atom [Mo–Cl (av.)= 2.32 Å, Mo–O = 1.7 Å]. The Mo atom is 0.41 Å above the basal plane. There is a longer Mo ⋯ Cl interaction of 2.9 Å in the octahedral position through the basal plane; if this were stronger (ca. 2.3 Å), then the structure would consist of dimers. The structure is in agreement with a gas electron-diffraction study, which found a C_4v square pyramid with Mo–O = 1.658(5) and Mo–Cl = 2.279(3)Å. It is also in agreement with the structures of WOCl₄, ReOCl₄, WSCl₄, and WSBr₄ where similar square pyramids are observed, but associated with different crystal symmetries.