Phosphorus pentafluoride, commonly written PF5, is a covalent phosphorus halide that exists as a colorless gas at ordinary temperature and pressure. The compound contains one phosphorus atom bonded to five fluorine atoms and is an example of a hypercoordinate molecule in which phosphorus forms five bonds. It is often discussed in introductory inorganic and physical chemistry because of its geometry and dynamic behavior.
Structure and bonding
PF5 adopts a trigonal bipyramidal arrangement: three fluorine atoms occupy equatorial positions and two occupy axial positions. The axial and equatorial P–F interactions differ in length and strength, and the molecule undergoes rapid exchange of these positions by a mechanism known as Berry pseudorotation. Modern bonding descriptions emphasize delocalized electron density over simple d-orbital models for this expanded coordination.
Preparation and reactions
PF5 can be produced by direct fluorination of phosphorus or by fluorination of phosphorus-containing precursors using suitable fluorinating agents. It is a strong Lewis acid and readily accepts a fluoride ion to form the hexafluorophosphate anion, PF6−, a chemically stable species widely used as a noncoordinating counterion. PF5 reacts with water and other protic reagents, undergoing hydrolysis to give phosphorus oxyfluoride species and hydrogen fluoride; these reactions are corrosive and potentially hazardous.
Related reagents used in phosphorus fluorination include oxygen difluoride. For information about the element at the center of PF5 see phosphorus, and for technical datasets consult sources listed at PF5 references.
Uses and significance
PF5 serves primarily as a reagent in inorganic synthesis and as a laboratory Lewis acid. Its ability to form PF6− makes it indirectly important in electrochemistry and in preparation of salts with weakly coordinating anions. PF5 and its derivatives have also been used in studies of molecular dynamics and spectroscopy because the molecule displays clear, well-understood examples of fluxional behavior.
Safety and handling
Because PF5 hydrolyzes to produce corrosive and toxic hydrogen fluoride and other phosphorus oxides, it must be handled under dry, inert atmosphere conditions with appropriate protective equipment. Storage and transport require gas-handling systems designed for reactive fluorinated gases. Laboratory-scale work typically uses small quantities inside gloveboxes or gas-tight apparatus.