Main Group Chemistry

6 Pages 1409 Words

Synthesis And Stroichiometries Of Polyhalide Ions Via Gravimetric And Potentiometric Analyses
Introduction

Halides are an important class of compounds, which have many different applications in chemistry. In most cases we come across halides in the –1 oxidation state (x-), however some of the heavier halogens are easily oxidised as their outermost electrons are only loosely held by weak forces, and thus can be found in the higher oxidation states. This is due to the halogens being bound to electronegative elements such as oxygen. Halogens themselves are electronegative and hence it is possible to form a wide range of interhalogen compounds.

It is possible to form positively and negatively charged interhalogen complexes. Negatively charged interhalogen complexes are called polyhalide ions. There are lots of examples of such anions containing only halogen atoms. There are formed by the addition of halogen molecules to simple halides. For example, the addition of F2 to Kbr might be expected to form the polyhalide anions [BrF2]-, [BrF4]-, [BrF6]-. Homoleptic polyhalides such as I3- can be formed from the addition of I2 to the simple ionic halide KI. In all of the interhalogen compounds, the heavier halogen that forms the central atom is most oxidised. While there are a wide-range of such compounds known, their structures can be worked out using VSEPR theory despite the fact that the bonding in such molecules is not of the localized 2-centre 2-electron type.

Stable complexes of these species can generally only be isolated in the presence large cations. One of the reasons for this is that the lattice energies of solids which have a large anion and a small cation such as Na+ tend to be low compared to that of the simple salt NaI. Because of this, recrystallisation of NaI3, for example from solutions of iodine tends to lead to the isolation of either unstable polyhalides, which decompose at room temperature or of sodium iodide.

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