Phosphine - Structure and Properties

Structure and Properties

PH₃ is a trigonal pyramidal molecule with C_3v molecular symmetry. The length of the P-H bond 1.42 Å, the H-P-H bond angles are 93.5°. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: CH₃PH₂, 1.10 D; (CH₃)₂PH, 1.23 D; (CH₃)₃P, 1.19 D. In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. The low dipole moment and almost orthogonal bond angles lead to the conclusion that in PH₃ 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. For this reason, the lone pair on phosphorus may be regarded as predominantly formed by the 3s orbital of phosphorus. 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). This electronic structure leads to a lack of nucleophilicity and an ability to form only weak hydrogen bonds.

The aqueous solubility of PH₃ is slight; 0.22 mL of gas dissolve in 1 mL of water. Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P-H bonds. It acts as neither an acid nor a base in water. Proton exchange proceeds via a phosphonium (PH₄+) ion in acidic solutions and via PH₂− at high pH, with equilibrium constants K_b = 4 × 10−28 and K_z = 41.6 × 10−29.