Triphenylphosphine oxide
Triphenylphosphine oxide (often abbreviated TPPO) is the organophosphorus compound with the formula OP(CH), also written as PhPO or PPhO (Ph = CH). This colourless crystalline compound is a common but potentially useful waste product in reactions involving triphenylphosphine. It is a popular reagent to induce the crystallizing of chemical compounds. Structure and properties. PhPO is a tetrahedral molecule related to POCl. The oxygen center is relatively basic. The rigidity of the backbone and the basicity of the oxygen center make this species a popular agent to crystallize otherwise difficult to crystallize molecules. This trick is applicable to molecules  that have acidic hydrogen atoms, e.g. phenols. Up to now, several modifications of PhPO have been found: For example, a monoclinic form crystalizes in the space group "P"2/"c" with Z = 4 and a = 15.066(1) Å, b = 9.037(2) Å, c = 11.296(3) Å, and β = 98.47(1)°. The orthorhombic modification crystallizes in the space group "Pbca" with Z = 4 and 29.089(3) Å, b = 9.1347(9), c = 11.261(1) Å. As a byproduct of organic synthesis. PhPO is a byproduct of many useful reactions in organic synthesis including the Wittig, Staudinger, and Mitsunobu reactions. It is also  formed when PPhCl is employed to convert alcohols into alkyl chlorides: Triphenylphosphine can be regenerated from the oxide by treatment with a variety of deoxygenation agents, such as phosgene or trichlorosilane/triethylamine: Triphenylphosphine oxide can be difficult to remove from reaction mixtures by means of chromatography. It is poorly soluble in hexane and cold diethyl ether. Trituration or chromatography of crude products with these solvents often leads to a good separation of triphenylphosphine oxide. Its removal is facilitated by conversion to its Mg(II) complex, which is poorly soluble in toluene or dichloromethane and can be filtered off. An alternative filtration  method where ZnCl(TPPO) is formed upon addition of ZnCl may be used with more polar solvents such as ethanol, ethyl acetate and tetrahydrofuran. Coordination chemistry. PhPO is a ligand for "hard" metal centers. A representative complex is the tetrahedral species NiCl(OPPh). PhPO is a common impurity in PPh. The oxidation of PPh by oxygen, including air, is catalyzed by many metal ions: