phosphine

Phosphine, PH3, is the phosphorus cousin of ammonia. If you slide one step down Group 15 — from nitrogen to phosphorus — and ask for the simple hydride, you get this colourless, highly toxic gas, traditionally said to smell of rotten fish or garlic (the pure gas is nearly odourless; the smell comes from impurities). It is the molecule that lets you compare, side by side, what changes as you go from the first-row element to the one below it.

PH3 has the same basic shape as ammonia — a trigonal pyramid with three P-H bonds and a lone pair — but the differences are revealing. The H-P-H angle is only about 94 degrees, much smaller than ammonia's roughly 107 degrees, because the larger phosphorus atom uses orbitals that are closer to pure p character with little hybridisation, so the bonds sit near right angles. Phosphine is a far weaker base than ammonia: its lone pair is in a larger, more diffuse orbital, held less eagerly and offered less readily to a proton, so PH3 is barely basic in water. It is also much more easily oxidised — pure phosphine can ignite in air, and impure samples often do spontaneously because of traces of diphosphine (P2H4).

Phosphine matters in several real corners of chemistry. It is used (often generated in situ from aluminium or zinc phosphide) as a fumigant to kill insects and rodents in stored grain. As a ligand its own importance is modest, but its organic derivatives — the tertiary phosphines such as triphenylphosphine, PPh3 — are among the most important ligands in all of organometallic and catalytic chemistry, where a soft, tunable phosphorus donor binds beautifully to soft transition metals. Phosphine also illustrates a recurring Group 15 theme: as you descend the group the hydride becomes a weaker base, less stable, and the central atom less keen to form strong multiple bonds.