Phosphorus Triiodide

Phosphorus triiodide is an inorganic compound composed of one phosphorus and three iodine atoms. Phosphorus forms two types of halides, phosphorus trihalides, and phosphorus pentahalides. The covalency of phosphorus in phosphorus trihalides is three and five in pentahalides. In this article, we will discuss the phosphorus triiodide structure PI3.

The chemical properties of phosphorus triiodide PI3 depend upon the electronic configuration and hybridization of the phosphorus.  The uses of phosphorus PI3 are different from the other phosphorus trihalide, due to its chemical and physical properties. The chemical representation of phosphorus triiodide can be answered by the same question as what is the formula for phosphorus triiodide.

What is the Chemical Formula for Phosphorus Triiodide?

The most common question asked while studying harmful phosphorus compounds in inorganic chemistry is that what is the formula for phosphorus triiodide? The answer to this question is PI3. 

Phosphorus Triiodide Structure PI3

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The structure of phosphorus triiodide contains three single sigma bonds between phosphorus and iodine and one lone pair. The bond length is 243 pico meters and the bond angle between the two bonds is 102 degrees celsius. Due to the repulsion between the lone pair and bond pair the angle is less than tetrahedral. Phosphorus triiodide structure PI3 represents the distorted tetrahedral shape. 

Preparation of Phosphorus Triiodide

Phosphorus triiodide is prepared by the free elements union method. Generally, iodine is mixed with the solution of white phosphorus in the neutral medium. Carbon disulfide is added in the preparation reaction of phosphorus triiodide. It provides a neutral medium for the reaction.

P4 + 6I2 → 4PI3

Another method for the preparation of phosphorus triiodide is the conversion of other phosphorus halides like phosphorus chloride and phosphorus bromide to the phosphorus triiodide by the addition of an iodide source (hydrogen iodide or metal iodide). The source of the iodine used in this reaction can be organic or inorganic.

PCl3 + 3HI → PI3 + 3HCl

Properties of Phosphorus Triiodide

Physical Properties of Phosphorus Triiodide PI3

• Phosphorus triiodide exists in solid form.

• It is generally found in the red colour.

• It is corrosive in nature.

• It acts as an irritant compound.

• The molecular weight of the phosphorus triiodide is 411.6872 g/mol.

• The density of phosphorus triiodide is 4.18 g/cm3.

• The boiling point of phosphorus triiodide is 200 degrees celsius.

• The melting point of the phosphorus triiodide is 61.2 degrees celsius.

Chemical Properties of Phosphorus Triiodide PI3

• The dipole moment of Phosphorus triiodide is nearly zero.

• The P-I bond is weak in nature.

• Phosphorus triiodide is an unstable molecule. Therefore, reactive in nature.

• Phosphorus triiodide reacts violently with water.

PI3 + 3H2O → H3PO3 + 3HI

• Phosphorus triiodide is a good reducing agent in nature.

• The standard enthalpy of formation of phosphorus triiodide is -46 KJ/mol.

• The NMR chemical shift for phosphorus triiodide is 178 ppm.

• Phosphorus triiodide reacts with methyl iodide in the presence of mercury and forms a complex (Trimethyl Phosphine Mercury) as a product.

PI3 + 3CH3I + Hg → [(CH3)3PHgI2]

Uses of Phosphorus Triiodide

The uses of phosphorus PI3 are discussed below:

• Phosphorus triiodide is widely used in organic chemistry branch laboratories for converting alkane halide to alkane iodide.

• It is used as various substitution reactions like the substitution of a hydroxyl group from alcohol.

• Phosphorus triiodide is used in the manufacturing of phosphoric acid.

• Phosphorus triiodide is used in the manufacturing of idopyrazines.

Did You Know? 

• Phosphorus triiodide can act as a nucleophile. Due to the presence of one lone pair, it can donate this pair to the electron-deficient compounds.

• PI3  can also act as an electrophile. Due to the presence of an empty d orbital, it can accept electrons from electron-rich compounds and expands its valency to 5.

• PI3  is highly toxic in nature.

• PI3  violently reacts with water and produces large amounts of heat.

• PI3 causes irritation to the eyes, skin, and respiratory system.