Phosphorus compounds are compounds containing the element phosphorus (P).
Phosphorus(V)
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The most prevalent compounds of phosphorus are derivatives of phosphate (PO43−), a tetrahedral anion.[1] Phosphate is the conjugate base of phosphoric acid, which is produced on a massive scale for use in fertilisers. Being triprotic, phosphoric acid converts stepwise to three conjugate bases:
- H3PO4 + H2O ⇌ H3O+ + H2PO4− Ka1 = 7.25×10−3
- H2PO4− + H2O ⇌ H3O+ + HPO42− Ka2 = 6.31×10−8
- HPO42− + H2O ⇌ H3O+ + PO43− Ka3 = 3.98×10−13
Phosphate exhibits a tendency to form chains and rings containing P-O-P bonds. Many polyphosphates are known, including ATP. Polyphosphates arise by dehydration of hydrogen phosphates such as HPO42− and H2PO4−. For example, the industrially important pentasodium triphosphate (also known as sodium tripolyphosphate, STPP) is produced industrially on by the megatonne by this condensation reaction:
- 2 Na2[(HO)PO3] + Na[(HO)2PO2] → Na5[O3P-O-P(O)2-O-PO3] + 2 H2O
Phosphorus pentoxide (P4O10) is the acid anhydride of phosphoric acid, but several intermediates between the two are known. This waxy white solid reacts vigorously with water.
With metal cations, phosphate forms a variety of salts. These solids are polymeric, featuring P-O-M linkages. When the metal cation has a charge of 2+ or 3+, the salts are generally insoluble, hence they exist as common minerals. Many phosphate salts are derived from hydrogen phosphate (HPO42−).
PCl5 and PF5 are common compounds. PF5 is a colourless gas and the molecules have trigonal bipyramidal geometry. PCl5 is a colourless solid which has an ionic formulation of PCl4+ PCl6−, but adopts the trigonal bipyramidal geometry when molten or in the vapour phase.[2] PBr5 is an unstable solid formulated as PBr4+Br−and PI5 is not known.[2] The pentachloride and pentafluoride are Lewis acids. With fluoride, PF5 forms PF6−, an anion that is isoelectronic with SF6. The most important oxyhalide is phosphorus oxychloride, (POCl3), which is approximately tetrahedral.
Before extensive computer calculations were feasible, it was thought that bonding in phosphorus(V) compounds involved d orbitals. Computer modeling of molecular orbital theory indicates that this bonding involves only s- and p-orbitals.[3]
Phosphorus(III)
editAll four symmetrical trihalides are well known: gaseous PF3, the yellowish liquids PCl3 and PBr3, and the solid PI3. These materials are moisture sensitive, hydrolysing to give phosphorous acid. The trichloride, a common reagent, is produced by chlorination of white phosphorus:
- P4 + 6 Cl2 → 4 PCl3
The trifluoride is produced from the trichloride by halide exchange. PF3 is toxic because it binds to haemoglobin.
Phosphorus(III) oxide, P4O6 (also called tetraphosphorus hexoxide) is the anhydride of P(OH)3, the minor tautomer of phosphorous acid. The structure of P4O6 is like that of P4O10 without the terminal oxide groups.
Phosphorus(I) and phosphorus(II)
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These compounds generally feature P–P bonds.[2] Examples include catenated derivatives of phosphine and organophosphines. Compounds containing P=P double bonds have also been observed, although they are rare.
Phosphides and phosphines
editPhosphides arise by reaction of metals with red phosphorus. The alkali metals (group 1) and alkaline earth metals can form ionic compounds containing the phosphide ion, P3−. These compounds react with water to form phosphine. Other phosphides, for example Na3P7, are known for these reactive metals. With the transition metals as well as the monophosphides there are metal-rich phosphides, which are generally hard refractory compounds with a metallic lustre, and phosphorus-rich phosphides which are less stable and include semiconductors.[2] Schreibersite is a naturally occurring metal-rich phosphide found in meteorites. The structures of the metal-rich and phosphorus-rich phosphides can be complex.
Phosphine (PH3) and its organic derivatives (PR3) are structural analogues of ammonia (NH3), but the bond angles at phosphorus are closer to 90° for phosphine and its organic derivatives. It is an ill-smelling, toxic compound. Phosphorus has an oxidation number of −3 in phosphine. Phosphine is produced by hydrolysis of calcium phosphide, Ca3P2. Unlike ammonia, phosphine is oxidised by air. Phosphine is also far less basic than ammonia. Other phosphines are known which contain chains of up to nine phosphorus atoms and have the formula PnHn+2.[2] The highly flammable gas diphosphine (P2H4) is an analogue of hydrazine.
Oxoacids
editPhosphorous oxoacids are extensive, often commercially important, and sometimes structurally complicated. They all have acidic protons bound to oxygen atoms, some have nonacidic protons that are bonded directly to phosphorus and some contain phosphorus - phosphorus bonds.[2] Although many oxoacids of phosphorus are formed, only nine are commercially important, and three of them, hypophosphorous acid, phosphorous acid, and phosphoric acid, are particularly important.
| Oxidation state | Formula | Name | Acidic protons | Compounds |
|---|---|---|---|---|
| +1 | HH2PO2 | hypophosphorous acid | 1 | acid, salts |
| +3 | H2HPO3 | phosphorous acid | 2 | acid, salts |
| +3 | HPO2 | metaphosphorous acid | 1 | salts |
| +3 | H3PO3 | (ortho)phosphorous acid | 3 | acid, salts |
| +4 | H4P2O6 | hypophosphoric acid | 4 | acid, salts |
| +5 | (HPO3)n | metaphosphoric acids | n | salts (n = 3,4,6) |
| +5 | H(HPO3)nOH | polyphosphoric acids | n+2 | acids, salts (n = 1-6) |
| +5 | H5P3O10 | tripolyphosphoric acid | 3 | salts |
| +5 | H4P2O7 | pyrophosphoric acid | 4 | acid, salts |
| +5 | H3PO4 | (ortho)phosphoric acid | 3 | acid, salts |
Nitrides
editThe PN molecule is considered unstable, but is a product of crystalline phosphorus nitride decomposition at 1100 K. Similarly, H2PN is considered unstable, and phosphorus nitride halogens like F2PN, Cl2PN, Br2PN, and I2PN oligomerise into cyclic Polyphosphazenes. For example, compounds of the formula (PNCl2)n exist mainly as rings such as the trimer hexachlorophosphazene. The phosphazenes arise by treatment of phosphorus pentachloride with ammonium chloride:
PCl5 + NH4Cl → 1/n (NPCl2)n + 4 HCl
When the chloride groups are replaced by alkoxide (RO−), a family of polymers is produced with potentially useful properties.[4]
Sulfides
editPhosphorus forms a wide range of sulfides, where the phosphorus can be in P(V), P(III) or other oxidation states. The three-fold symmetric P4S3 is used in strike-anywhere matches. P4S10 and P4O10 have analogous structures.[5] Mixed oxyhalides and oxyhydrides of phosphorus(III) are almost unknown.
Organophosphorus compounds
editCompounds with P-C and P-O-C bonds are often classified as organophosphorus compounds. They are widely used commercially. The PCl3 serves as a source of P3+ in routes to organophosphorus(III) compounds. For example, it is the precursor to triphenylphosphine:
- PCl3 + 6 Na + 3 C6H5Cl → P(C6H5)3 + 6 NaCl
Treatment of phosphorus trihalides with alcohols and phenols gives phosphites, e.g. triphenylphosphite:
- PCl3 + 3 C6H5OH → P(OC6H5)3 + 3 HCl
Similar reactions occur for phosphorus oxychloride, affording triphenylphosphate:
- OPCl3 + 3 C6H5OH → OP(OC6H5)3 + 3 HCl
See also
editReferences
edit- ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
- ^ a b c d e f Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
- ^ Kutzelnigg, W. (1984). "Chemical Bonding in Higher Main Group Elements" (PDF). Angew. Chem. Int. Ed. Engl. 23 (4): 272–295. doi:10.1002/anie.198402721. Archived from the original (PDF) on 2020-04-16. Retrieved 2009-05-24.
- ^ Mark, J. E.; Allcock, H. R.; West, R. "Inorganic Polymers" Prentice Hall, Englewood, NJ: 1992. ISBN 0-13-465881-7.
- ^ Heal, H. G. "The Inorganic Heterocyclic Chemistry of Sulfur, Nitrogen, and Phosphorus" Academic Press: London; 1980. ISBN 0-12-335680-6.