Crystal structure of tetraphenylarsonium cis-tetracarbonyl(chloromercurio)(dichloromercurio)ferrate

The crystal structure of the title compound has been determined by X-ray diffraction (heavy-atom) methods and refined by least squares to R 0.059 for 2 917 observed reflections. Crystals are triclinic P, a = 11.848(3), b= 13.524(4), c= 11.624(3)A, α= 123.87(2), β= 94.01 (2), γ= 90.91 (2)°, Z= 2. Within the anion, the two mercury atoms are bonded cis in the co-ordination sphere of the six-co-ordinate iron atom [Fe–Hg 2.560(3) and 2.516(4)A, Hg–Fe–Hg 80.3(1)°]. The anion lies close to a centre of symmetry and is linked to its inversion image by one of the chlorine atoms. The reduction in the Hg–Fe–Hg angle from the regular octahedral value is shown to be a consequence of the more extended Fe–Hg relative to the Fe–CO bonds, rather than to Hg ⋯ Hg interactions.