phosphines

Phosphines are a class of organophosphorus compounds with the formula PR3, in which a phosphorus atom bears a lone pair and three substituents R, which may be alkyl or aryl, or hydrogen. They include primary phosphines (RPH2), secondary phosphines (R2PH), and tertiary phosphines (R3P); the most widely used are tertiary phosphines such as triphenylphosphine (PPh3).

The phosphorus center in phosphines is trivalent and typically adopts a pyramidal geometry due to the lone pair. Substituents influence both steric and electronic properties, affecting how strongly the ligand donates electron density to metals and how accessible the metal center is. Phosphines are primarily sigma-donor ligands for transition metals and can modulate catalytic activity and selectivity in homogeneous catalysis. Bulkier phosphines can also alter steric environments around the metal.

Synthesis of tertiary phosphines commonly involves the generation of PR3 from phosphorus trichloride (PCl3) using organometallic reagents such as Grignard reagents or organolithium compounds, followed by workup. Primary and secondary phosphines can be prepared by related routes or by functionalization of related phosphorus intermediates. Phosphines can be oxidized to phosphine oxides (R3P=O), and many are air-sensitive to varying degrees; some are relatively air-stable under ordinary conditions, but proper handling under inert atmosphere is common in the laboratory.

Uses and relevance: as ligands in coordination chemistry and catalysis, phosphines support a wide range of reactions, including hydrogenation, hydroformylation, and cross-coupling. Well-known examples include triphenylphosphine (PPh3) and bulky phosphines such as PCy3, whose steric and electronic properties are tailored to optimize reactivity and selectivity in specific catalytic cycles.

Safety: many phosphines are toxic or malodorous and can pose hazards if inhaled or ingested. Phosphine gas (PH3) is highly toxic. They can ignite under certain conditions and oxidize to phosphine oxides; handling typically requires appropriate precautions and, for air-sensitive examples, inert-atmosphere techniques.