Arsenic pentachloride

Arsenic pentachloride is a chemical compound of arsenic and chlorine. This compound was first prepared in 1976 through the UV irradiation of arsenic trichloride, AsCl₃, in liquid chlorine at −105 °C. AsCl₅ decomposes at around −50 °C. The structure of the solid was finally determined in 2001. AsCl₅ is similar to phosphorus pentachloride, PCl₅ in having a trigonal bipyramidal structure where the equatorial bonds are shorter than the axial bonds (As-Cl_eq = 210.6 pm, 211.9 pm; As-Cl_ax= 220.7 pm).

The pentachlorides of the elements above and below arsenic in group 15, phosphorus pentachloride and antimony pentachloride are much more stable and the instability of AsCl₅ appears anomalous. The cause is believed to be due to incomplete shielding of the nucleus in the elements following the first transition series (i.e. gallium, germanium, arsenic, selenium and bromine) which leads to stabilisation of their 4s electrons making them less available for bonding. This effect has been termed d-block contraction and is similar to the f-block contraction normally termed the lanthanide contraction.