Phosphine, arsane, and stibine

Key points: Unlike liquid ammonia, liquid phosphine, arsane, and stibane do not associate through hydrogen bonding; their much more stable alkyl and aryl analogues are useful soft ligands. In contrast to the commanding role that ammonia plays in nitrogen chemistry, the highly poisonous hydrides of the heavier nonmetallic elements of Group 15 (particularly phosphine, PH3 , and arsane, AsH3 ) are of minor importance in the chemistry of their respective elements. Both phosphine and arsane are used in the semiconductor industry to dope Si or to prepare other semiconductor compounds, such as GaAs, by chemical vapour deposition. These thermal decomposition reactions reflect the positive Gibbs energy of formation of these hydrides. The commercial synthesis of PH3 uses the disproportionation of white phosphorus in basic solution:

Arsane and stibane may be prepared by the protolysis of com pounds that contain an electropositive metal in combination with arsenic or antimony:

Phosphine and arsane are poisonous gases that readily ignite in air, but the much more stable organic derivatives PR₃ and AsR₃ (R = alkyl or aryl groups) are widely used as ligands in metal coordination chemistry. In contrast with the hard donor properties of ammonia and alkylamine ligands, the organophosphines and organoarsanes, such as P (C₂H₅)₃ and As(C₆H₅)₃ , are soft ligands and are therefore often incorporated into metal complexes having central metal atoms in low oxidation states. The stability of these complexes correlates with the soft acceptor nature of the metals in low oxidation states, and the stability of soft-donor soft-acceptor combinations (Section 4.12). All the Group 15 hydrides are pyramidal, but the bond angle decreases down the group:

The large change in bond angle has been attributed to a decrease in the extent of sp³ hybridization from NH₃ to SbH₃ but is more likely to be due to steric effects. The E-H bonding pairs of electrons will repel each other. This repulsion is greatest when the central element, E, is small, as in NH₃, and the H atoms will be as far away from each other as possible in a near-tetrahedral arrangement. As the size of the central atom is increased down the series, the repulsion between bonding pairs decreases and the bond angle is close to 90º. It is evident from the boiling points plotted in Fig. 10.6 that PH₃, AsH₃, and SbH₃ are subject to little, if any, hydrogen bond ing with themselves; however, PH₃ and AsH₃ can be protonated by strong acids, such as HI, to form phosphonium and arsoniumions, PH₄⁺ and AsH₄⁺, respectively.

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مواضيع ذات صلة

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides

Nitrogen activation

Microporous solids

Aluminosilicates

Compounds with nitrogen

Oxides of germanium, tin, and lead

Compounds of carbon with oxygen and sulfur

Tin and lead halides