tertiary phosphine ligands
/ phosphine (FOS-feen) /
If carbon monoxide is the loyal, predictable employee that every metal complex hires, the tertiary phosphine is the adjustable tool whose dial a chemist can turn. A tertiary phosphine, written PR3, is a phosphorus atom carrying three organic groups and one lone pair, and that lone pair is what binds it to a metal. The beauty is that by choosing the three R groups you can tune both how electron-rich and how bulky the ligand is, almost independently, giving you a knob for the metal's electronics and a knob for its geometry.
How it bonds is familiar: the phosphorus lone pair donates into the metal (a sigma donation), and phosphines also accept a little electron density back from the metal, so they are mild pi-acceptors, though far weaker than CO. The two tuning knobs are these. Electronically, electron-donating alkyl groups like the three ethyls in PEt3 make the phosphorus a strong donor, while electron-withdrawing groups, especially fluorinated or the trifluorophosphite-like P(OR)3 and the extreme PF3, make it a much weaker donor and stronger acceptor. Sterically, big groups make the ligand bulky, and the size is captured by the Tolman cone angle, the apex angle of a cone drawn from the metal that just encloses the ligand. A small PMe3 has a cone angle near 118 degrees; a hefty tricyclohexylphosphine or triphenylphosphine spreads to 145 to 170 degrees, elbowing other ligands aside.
This independent tunability is why phosphines are everywhere in catalysis. Switch from a small to a bulky phosphine and you can force a metal to drop a ligand and open a coordination site, or favour one geometry over another; switch from a donating to a withdrawing phosphine and you change how readily the metal undergoes oxidative addition. Wilkinson's catalyst carries triphenylphosphines; modern cross-coupling catalysts owe much of their success to specially designed bulky, electron-rich phosphines. The practical caveat is that many alkylphosphines are air-sensitive and oxidize to useless phosphine oxides, so they are handled under inert atmosphere.
Compare PMe3 and P(C6H11)3 (tricyclohexylphosphine). Both are strong electron donors, but PMe3 has a cone angle near 118 degrees while the cyclohexyl version is around 170 degrees. Swapping one for the other changes nothing electronically yet can force a metal to release a second ligand purely on size.
Phosphines let you tune sterics and electronics nearly independently, a rare and useful freedom.
Phosphines are only weak pi-acceptors compared with CO; their acceptor strength comes mainly from phosphorus-substituent antibonding orbitals, not from old textbook claims of empty phosphorus d orbitals, which are now considered largely incorrect.