Quaternary phosphonium and arsonium compounds:

Some quaternary phosphonium and arsonium compounds have found extensive use in the extraction of simple acid radicals, oxyanions of metals, anionic inorganic coordination complexes involving organic ligands. These extractants behave as strong electrolytes and are usually water soluble. The metal complexes with a well-known stoichiometry are formed when reagent is added to a neutral or weakly acidic solution and they are transformed into a slightly polar organic solvent such as chloroform or methyl isobutyl ketone. Apart from the effect of anion on the extractability of the ion pair, the size of the organic cation bears markedly on the extractive property. Another property which prominently affects the extraction is the nature of the diluent. A very typical example is the extraction of MnO ^-₄  or ReO₄^-  in chloroform formed by tetraphenylarsonium chloride. The extracting Mn(VII) species is proposed to be (C₆H₅)₄As+.MnO^-₄ . Some dimerization of the monomer in the organic phase also takes place.