Phosphine (PH3) is a Lewis base. The phosphorus atom carries a lone pair of electrons that it can donate to electron-poor species, which is the defining behavior of a Lewis base. While phosphorus compounds can sometimes act as Lewis acids under unusual circumstances, PH3 in its normal form functions squarely as an electron pair donor.
Why PH3 Acts as a Lewis Base
A Lewis base is any species that donates a pair of electrons. Phosphorus has five valence electrons. In PH3, three of those electrons are used to form bonds with the three hydrogen atoms, leaving two electrons as a lone pair sitting on the phosphorus. That lone pair is available to be shared with an electron-accepting species, making PH3 a Lewis base.
This is the same basic logic that makes ammonia (NH3) a Lewis base. Nitrogen also has a lone pair it can donate. The difference is in how tightly that lone pair is held and how readily the molecule donates it.
The Unusual Structure Behind PH3’s Basicity
PH3 has a peculiar electronic structure that sets it apart from ammonia. In NH3, nitrogen’s orbitals mix together (hybridize) into four equivalent lobes, three for bonding and one for the lone pair. Phosphorus in PH3 largely skips this step. Instead, it uses its three p orbitals to bond with hydrogen and keeps the lone pair tucked in the lower-energy s orbital.
You can see this in the bond angle. NH3 has a bond angle of about 107°, close to the ideal tetrahedral angle of 109.5°. PH3 has a much tighter bond angle of 93.5°, barely above the 90° you’d expect from pure p orbitals doing all the bonding. This confirms that the lone pair sits mostly in the s orbital rather than mixing into a hybrid.
This matters because an s orbital holds its electrons closer to the nucleus and is less directional than a hybrid orbital. The lone pair in PH3 is therefore less “available” for donation compared to the lone pair in NH3, where the hybrid orbital points outward more aggressively. PH3 is still a Lewis base, but a weaker one than ammonia.
PH3 Compared to Ammonia as a Lewis Base
Both PH3 and NH3 are Lewis bases, but ammonia is significantly stronger. Ammonia readily participates in acid-base reactions, forming bonds with protons and metal ions with ease. Phosphine is less basic and less reactive under similar conditions. The s-character of the phosphorus lone pair is one reason. Another is that phosphorus is a larger atom, so its lone pair is more diffuse and overlaps less effectively with acceptor orbitals.
That said, phosphine and its relatives are extremely important in chemistry precisely because they donate electron pairs. Phosphine ligands form coordination compounds with transition metals. Complexes like PtCl₂(PH₃)₂ exist because PH3 donates its lone pair to the platinum center. In practice, chemists more commonly use phosphines with organic groups attached (like triphenylphosphine) because those are stronger donors and easier to handle, but the principle is the same.
Could PH3 Ever Act as a Lewis Acid?
Phosphorus compounds are generally used as Lewis bases. P(III) species like PH3, with their accessible lone pair, are textbook electron donors. However, phosphorus-based Lewis acidity does exist in specialized chemistry. Phosphenium cations, first discovered in 1964, are positively charged phosphorus species that can accept electrons. These are fundamentally different from PH3, though. They carry a positive charge and lack the lone pair that defines phosphine’s chemistry.
For PH3 itself, there is no meaningful Lewis acid behavior. It does not have a low-energy empty orbital ready to accept an electron pair. If you encounter PH3 on an exam or in a textbook, the answer is straightforward: it is a Lewis base.
PH3 as a Brønsted-Lowry Base and Acid
In the Brønsted-Lowry framework, PH3 is a very weak base. It can accept a proton to form PH₄⁺, but it does so reluctantly compared to ammonia. It can also, in principle, lose a proton to act as an acid, but this requires extreme conditions. The pKa of PH3 is around 29 in water, meaning it essentially never gives up a hydrogen under normal circumstances. For context, water has a pKa of about 15.7, so PH3 is roughly 13 orders of magnitude less acidic than water.
In the Lewis acid-base framework, which focuses purely on electron pair donation and acceptance, the classification is cleaner. PH3 has a lone pair, it donates that lone pair, and it is a Lewis base.