Phosphorus trifluoride
Phosphorus trifluoride (PF₃) is an inorganic compound consisting of a central phosphorus atom bonded to three fluorine atoms in a trigonal pyramidal geometry, with a molecular weight of 87.97 g/mol. It appears as a colorless, odorless gas at standard conditions, with a density of 3.91 g/L, a melting point of −151.5 °C, and a boiling point of −101.8 °C.[1] PF₃ is highly reactive toward moisture, undergoing slow hydrolysis to form phosphorous acid and hydrogen fluoride, and it is notable for its role as a ligand analogous to carbon monoxide in transition metal coordination chemistry.[1] The compound is typically synthesized via halogen exchange reactions, such as the fluorination of phosphorus trichloride (PCl₃) with hydrogen fluoride or other fluoride sources under controlled conditions to yield PF₃ and byproducts like HCl.[2] Alternative industrial preparations involve the direct reaction of elemental phosphorus with anhydrous hydrogen fluoride at elevated temperatures (180–220 °C) under autogenous pressure.[3] Due to its π-acceptor properties, PF₃ forms stable complexes with metals like iron, nickel, and platinum, often substituting for CO in carbonyl compounds, as demonstrated in the preparation of (PF₃)₃Fe(CO)₂ from Fe(CO)₅.[4] PF₃ is used as a ligand in organometallic catalysis and coordination compounds, where its electron-withdrawing nature influences reaction selectivity similar to CO but with enhanced stability in some systems,[5] and in semiconductor processing, such as plasma etching and ion implantation, leveraging its reactivity to form volatile byproducts like CFₓ species that aid in material patterning.[6] However, its handling requires stringent safety measures owing to acute toxicity; inhalation causes severe respiratory distress, and contact leads to corrosive burns on skin and eyes, with an ACGIH threshold limit value of 2.5 mg/m³.[7]Structure and bonding
Molecular geometry
Phosphorus trifluoride adopts a trigonal pyramidal molecular geometry, featuring the central phosphorus atom at the apex bonded to three fluorine atoms that form the base of the pyramid.[8] This arrangement arises from the presence of a lone pair on the phosphorus atom, which occupies one vertex of an idealized tetrahedron around the central atom. The geometry aligns with the Valence Shell Electron Pair Repulsion (VSEPR) theory prediction for an AX<sub>3</sub>E<sub>1</sub> electron domain configuration, where the lone pair exerts greater repulsion than the bonding pairs, distorting the structure from trigonal planar.[9] The F–P–F bond angle measures approximately 96.3°, significantly smaller than the tetrahedral ideal of 109.5° due to enhanced repulsion from the lone pair on phosphorus.[8] The P–F bond length is 1.56 Å.[10] This asymmetry imparts a dipole moment of 1.03 D to the molecule, with the negative end directed toward the fluorine atoms.[11]Electronic properties
Phosphorus trifluoride (PF₃) features a central phosphorus atom that undergoes sp³ hybridization in its valence bond description, forming four equivalent hybrid orbitals. Three of these orbitals are utilized to create σ-bonds with the fluorine atoms, while the fourth hybrid orbital houses the lone pair of electrons on phosphorus. This hybridization arrangement accounts for the molecule's overall electron domain geometry being tetrahedral, with the lone pair influencing the molecular shape. In terms of bonding, the lone pair on phosphorus enables PF₃ to function as a σ-donor ligand by donating electron density into the σ-acceptor orbital of a coordinated metal center. Additionally, PF₃ exhibits π-acceptor capability through its empty d-orbitals on the phosphorus atom, which can accept electron density from filled metal d-orbitals via π-backbonding. This dual donor-acceptor behavior is particularly pronounced in PF₃ compared to other phosphines, owing to the electronegative fluorine substituents that lower the energy of the phosphorus-based acceptor orbitals. The formal charge analysis of the Lewis structure reveals zero formal charge on the phosphorus atom and each fluorine atom, consistent with the octet satisfaction and equal sharing assumption in single P-F bonds.[12][13] As a ligand in transition metal complexes, the π-backbonding interaction with PF₃ strengthens its binding to low-oxidation-state metals, rendering it a strong π-acceptor comparable to carbon monoxide (CO). This similarity arises from the effective overlap between metal d-orbitals and the low-lying acceptor orbitals of PF₃, which enhances metal-ligand π-bonding and stabilizes electron-rich complexes. The first ionization energy of PF₃ is 11.65 ± 0.07 eV, corresponding to the removal of an electron from the highest occupied molecular orbital, which is predominantly the phosphorus lone pair orbital.[14]Physical properties
Thermodynamic data
Phosphorus trifluoride (PF₃) is a colorless, reactive gas with a molar mass of 87.97 g/mol.[15] Its density is 3.91 g/L at standard temperature and pressure (STP).[1] The compound exhibits low phase transition temperatures, with a melting point of −151.5 °C and a boiling point of −101.8 °C, reflecting its volatility as a gas under ambient conditions.[1] The standard enthalpy of formation (ΔH_f°) for gaseous PF₃ is −958.44 kJ/mol at 298 K.[16] Thermodynamic functions at 298 K include a constant-pressure heat capacity (C_p) of 58.7 J/mol·K and a standard entropy (S°) of 273.1 J/mol·K.[16] These values are derived from spectroscopic data and equilibrium measurements, with heat capacity modeled using the Shomate equation for temperatures from 298 K to 6000 K: C_p^\circ = A + B t + C t^2 + D t^3 + \frac{E}{t^2} where t = T/1000 (T in K), and parameters for 298–1000 K are A = 39.66369, B = 110.8434, C = −108.4355, D = 37.88286, E = −0.480131 (in J/mol·K).[16] PF₃ has a critical temperature of 271 K (−2.05 °C) and a critical pressure of 4.33 MPa, indicating moderate conditions for liquefaction beyond its boiling point.[17] Its vapor pressure follows a typical curve for low-boiling fluorides, rapidly increasing above the boiling point to reach atmospheric pressure at −101.8 °C, though exact equations are available in thermochemical compilations for engineering applications.[18]| Property | Value | Conditions | Source |
|---|---|---|---|
| Molar mass | 87.97 g/mol | - | NIST Webbook[15] |
| Density | 3.91 g/L | STP | ChemicalBook[1] |
| Melting point | −151.5 °C | 1 atm | ChemicalBook[1] |
| Boiling point | −101.8 °C | 1 atm | ChemicalBook[1] |
| ΔH_f° | −958.44 kJ/mol | 298 K, gas | NIST-JANAF[16] |
| C_p | 58.7 J/mol·K | 298 K, gas | NIST-JANAF[16] |
| S° | 273.1 J/mol·K | 298 K, gas | NIST-JANAF[16] |
| Critical temperature | 271 K | - | LookChem[17] |
| Critical pressure | 4.33 MPa | - | LookChem[17] |