![]() ![]() Proton exchange proceeds via a phosphonium ( PH + 4) ion in acidic solutions and via phosphanide ( PH − 2) at high pH, with equilibrium constants K b = 4 ×10 −28 and K a = 41.6 ×10 −29. It is technically amphoteric in water, but acid and base activity is poor. ![]() Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P−H bonds. The aqueous solubility of PH 3 is slight 0.22 cm 3 of gas dissolves in 1 cm 3 of water. This electronic structure leads to a lack of nucleophilicity in general and lack of basicity in particular (p K aH = –14), as well as an ability to form only weak hydrogen bonds. The upfield chemical shift of the phosphorus atom in the 31P NMR spectrum accords with the conclusion that the lone pair electrons occupy the 3s orbital (Fluck, 1973). For this reason, the lone pair on phosphorus may be regarded as predominantly formed by the 3s orbital of phosphorus. The low dipole moment and almost orthogonal bond angles lead to the conclusion that in PH 3 the P−H bonds are almost entirely pσ(P) – sσ(H) and phosphorus 3s orbital contributes little to the bonding between phosphorus and hydrogen in this molecule. ![]() In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: CH 3PH 2, 1.10 D (CH 3) 2PH, 1.23 D (CH 3) 3P, 1.19 D. The length of the P−H bond is 1.42 Å, the H−P−H bond angles are 93.5 °. PH 3 is a trigonal pyramidal molecule with C 3 v molecular symmetry. The gas PH 3 was named "phosphine" by 1865 (or earlier). The name "phosphine" was first used for organophosphorus compounds in 1857, being analogous to organic amines ( NR 3). Calcium phosphide (nominally Ca 3P 2) produces more P 2H 4 than other phosphides because of the preponderance of P-P bonds in the starting material. He considered diphosphine's formula to be PH 2, and thus an intermediate between elemental phosphorus, the higher polymers, and phosphine. In 1844, Paul Thénard, son of the French chemist Louis Jacques Thénard, used a cold trap to separate diphosphine from phosphine that had been generated from calcium phosphide, thereby demonstrating that P 2H 4 is responsible for spontaneous flammability associated with PH 3, and also for the characteristic orange/brown color that can form on surfaces, which is a polymerisation product. Perhaps because of its strong association with elemental phosphorus, phosphine was once regarded as a gaseous form of the element, but Lavoisier (1789) recognised it as a combination of phosphorus with hydrogen and described it as phosphure d'hydrogène (phosphide of hydrogen). Philippe Gengembre (1764–1838), a student of Lavoisier, first obtained phosphine in 1783 by heating white phosphorus in an aqueous solution of potash (potassium carbonate). Phosphine, PH 3, is the smallest of the phosphines and the smallest of the phosphanes. Phosphanes are saturated phosphorus hydrides of the form P nH n+2, such as triphosphane. They have the general formula PH 3− nR n. Phosphines are compounds that include PH 3 and the organophosphines, which are derived from PH 3 by substituting one or more hydrogen atoms with organic groups. Phosphine has a trigonal pyramidal structure. ![]() Phosphine is a highly toxic respiratory poison, and is immediately dangerous to life or health at 50 ppm. With traces of P 2H 4 present, PH 3 is spontaneously flammable in air ( pyrophoric), burning with a luminous flame. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting fish, due to the presence of substituted phosphine and diphosphane ( P 2H 4). Phosphine ( IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula PH 3, classed as a pnictogen hydride. ![]()
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