Phosphorus trifluoride
# Phosphorus trifluoride

Phosphorus trifluoride (formula $PF_3),$ is a colorless and odorless gas. It is highly toxic and reacts slowly with water. Its main use is as a ligand in metal complexes. As a ligand, it parallels carbon monoxide in metal carbonyls, and indeed its toxicity is due to its binding with the iron in blood hemoglobin in a similar way to carbon monoxide.

## Physical properties

Phosphorus trifluoride has an F−P−F bond angle of approximately 96.3°. Gaseous $PF_3$ has a standard enthalpy of formation of −945 kJ/mol (−226 kcal/mol). The phosphorus atom has a nuclear magnetic resonance chemical shift of 97 ppm (downfield of $H_3PO_4).$

## Properties

Phosphorus trifluoride hydrolyzes especially at high pH, but it is less hydrolytically sensitive than phosphorus trichloride. It does not attack glass except at high temperatures, and anhydrous potassium hydroxide may be used to dry it with little loss. With hot metals, phosphides and fluorides are formed. With Lewis bases such as ammonia addition products (adducts) are formed, and $PF_3$ is oxidized by oxidizing agents such as bromine or potassium permanganate.

As a ligand for transition metals, $PF_3$ is a strong π-acceptor. It forms a variety of metal complexes with metals in low oxidation states. $PF_3$ forms several complexes for which the corresponding CO derivatives (see metal carbonyl) are unstable or nonexistent. Thus, $Pd(PF_3)_4$ is known, but $Pd(CO)_4$ is not. Such complexes are usually prepared directly from the related metal carbonyl compound, with loss of CO. However, nickel metal reacts directly with $PF_3$ at 100 °C under 35 MPa pressure to form $Ni(PF_3)_4,$ which is analogous to $Ni(CO)_4.$ $Cr(PF_3)_6,$ the analogue of $Cr(CO)_6,$ may be prepared from dibenzenechromium:

    $Cr(C_6H_6)_2$ + 6 $PF_3$ → $Cr(PF_3)_6$ + 2 $C_6H_6$

| Ball-and-stick model of $[Pt(PF_3)_4]$ | Space-filling model of $[Pt(PF_3)_4]$ |
| - | - |

## Preparation

Phosphorus trifluoride is usually prepared from phosphorus trichloride via halogen exchange using various fluorides such as hydrogen fluoride, calcium fluoride, arsenic trifluoride, antimony trifluoride, or zinc fluoride:

    2 $PCl_3$ + 3 $ZnF_2$ → 2 $PF_3$ + 3 $ZnCl_2$

## Biological activity

Phosphorus trifluoride is similar to carbon monoxide in that it is a gas which strongly binds to iron in hemoglobin, preventing the blood from absorbing oxygen.

## Precautions

$PF_3$ is highly toxic, comparable to phosgene.

