pH4

PH4 is a chemical formula that can refer to several related hydrogen–phosphorus species. In practical terms, the most significant interpretation is the phosphonium cation PH4+, which forms when the tertiary base phosphine PH3 is protonated. PH4+ adopts a tetrahedral geometry with four equivalent P–H bonds and is a common motif in inorganic and organophosphorus chemistry. It forms salts with various anions, such as BF4-, PF6-, and SbF6-, especially in strongly acidic media or fluorinated solvents. The cation is often described as the phosphorus analogue of ammonium.

The neutral molecule PH4 is not known as a stable species under ordinary conditions. Achieving a neutral, tetrahydrogenated phosphorus atom would require phosphorus to carry an unusually low oxidation state, and as a result PH4 is not observed as a isolable compound. In theoretical studies, neutral PH4 may appear as a high-energy or transient species in gas phase or under special conditions, but it has not been characterized as a stable molecule.

PH4 can also be encountered in discussions outside pure chemistry, where the notation might reflect a typographical shorthand for pH 4, indicating an acidic solution with a pH value of four. In chemical literature, however, PH4 more reliably refers to the phosphonium cation or to the general concept of phosphorus–hydrogen bonding in tetrahydrogenated contexts, rather than to a widely used neutral compound.