Electronic structure: why phosphine differs from ammonia
Although phosphine ($PH_3$) and ammonia ($NH_3$) molecules appear to be complete analogs in composition, their chemical behavior differs cardinally. The main reason lies in the structural features of the electron shells of the central atoms.
In the ammonia molecule, the lone pair of electrons of the nitrogen atom is involved in hybridization. It forms a hybridized $sp^3$ orbital, which has a clear spatial orientation toward a potential proton acceptor. Due to this, the bond angle in ammonia is about $107^\circ$, and the substance itself is able to easily accept a hydrogen ion, exhibiting pronounced basic properties.
In the phosphine molecule, the situation is different. The lone electron pair of the phosphorus atom is localized on the unhybridized $s$ orbital. This electron cloud has a spherical shape and lacks spatial orientation. As a result, it is extremely difficult for the phosphorus atom to act as an electron pair donor for proton attachment. The absence of hybridization of the binding orbitals is confirmed by the value of the bond angle in the $PH_3$ molecule, which is approximately $93^\circ$ (i.e., close to $90^\circ$). This is precisely why the basic properties of phosphine are negligibly weak compared to ammonia.
Acid-base interactions and phosphonium salts
Due to the specifics of its electronic structure, phosphine dissolves poorly in water and is completely incapable of creating an alkaline environment in it. Salt-formation reactions involving this gas proceed with extreme difficulty and have strict limitations.
Phosphine enters into addition reactions exclusively with gaseous hydrogen halides. Upon contact with them, crystalline compounds—phosphonium salts—are formed. A classic example of such an interaction is the reaction with hydrogen bromide:
$$PH_3 + HBr \rightleftarrows PH_4Br$$
However, the resulting salts, such as phosphonium bromide ($PH_4Br$), are completely unstable in aqueous solutions. When attempting to dissolve them, complete and irreversible hydrolysis occurs. The salt is instantly destroyed by water with the abundant release of the initial gas — phosphine:
$$PH_4Br + H_2O = PH_3\uparrow + H_3O^+ + Br^-$$
This feature makes the existence of the phosphonium cation in aqueous media impossible.
Metal phosphides and their hydrolysis
Phosphorus is capable of reacting with active metals, forming binary compounds called phosphides. In these substances, phosphorus is also in the -3 oxidation state.
Like phosphonium salts, metal phosphides undergo complete hydrolysis upon interaction with water. As a result of this reaction, the corresponding metal hydroxide is formed and gaseous phosphine is released. For example, the hydrolysis of calcium phosphide is described by the following equation:
$$Ca_3P_2 + 6H_2O = 3Ca(OH)_2 + 2PH_3\uparrow$$
Similarly, phosphides are destroyed under the action of dilute acid solutions (e.g., sulfuric acid), which also leads to the intensive evolution of phosphine.
Redox properties
While the basic properties of phosphine are weakly expressed, its reducing ability, by contrast, is extremely high. In solutions, phosphine is a much stronger reducing agent than ammonia.
Phosphine is easily oxidized by various reagents, including potassium permanganate ($KMnO_4$). These reactions actively proceed in both acidic and alkaline environments. Upon oxidation in an excess of alkali, phosphorus transitions from the -3 oxidation state to the highest +5 oxidation state, forming phosphates, while permanganate is reduced to manganate:
$$PH_3 + 8KMnO_4 + 11KOH = K_3PO_4 + 8K_2MnO_4 + 7H_2O$$
Such chemical activity causes the low stability of phosphine in the presence of oxidizing agents and emphasizes its status as a powerful reducing agent.
To remember the difference between hydrogen compounds: "Ammonia is Active (has $sp^3$ hybridization), while Phosphine is Fictitious (no hybridization, electron pair is hidden in the $s$-sphere)."
Class and exam questions
Why doesn't an aqueous solution of phosphine turn indicators alkaline?
Unlike ammonia, phosphine practically does not react with water to form hydroxide ions. Its lone electron pair is located on the $s$ orbital and is unable to efficiently attach protons from water.
Is it possible to obtain phosphonium sulfate by passing the gas through a sulfuric acid solution?
No, the salt-formation reaction for phosphine is only possible with hydrogen halides in the gas phase. In aqueous solutions, phosphonium salts instantly hydrolyze.
How to correctly write the decomposition reaction of calcium phosphide by acid?
The process is analogous to hydrolysis: the phosphide reacts with a dilute acid (e.g., $H_2SO_4$) to form a metal salt (calcium sulfate) and release gaseous phosphine.
What else to study on this topic
- Oxidation of phosphine by potassium permanganate in an acidic medium (reaction equation)
- Interaction of phosphides with dilute acids (using sulfuric acid as an example)
- Reasons for the lower thermodynamic stability of phosphine compared to ammonia
- Detailed analysis of the electronic structure of the phosphorus atom without orbital hybridization
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