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Trihydrides of the Group 15 Elements

Module by: Andrew R. Barron. E-mail the author

All five of the Group 15 elements form hydrides of the formula EH3. Table 1 lists the IUPAC names along with those in more common usage.

Table 1: Traditional and IUPAC (International Union of Pure and Applied Chemistry) names for the Group 15 hydrides.
Compound Traditional name IUPAC name
NH3 Ammonia Azane
PH3 Phosphine Phosphane
AsH3 Arsine Arsane
SbH3 Stibine Stibane
BiH3 Bismuthine Bismuthane

The boiling point and melting point increase increases going down the Group (Table 2) with increased molecular mass, with the exception of NH3 whose anomalously high melting and boiling points (Figure 1) are a consequence of strong N-H...H hydrogen bonding. A similar (and stronger) effect is observed for the Group 16 hydrides (H2E).

Table 2: Selected physical properties of Group 15 hydrides.
Compound Mp (°C) Bp (°C) ΔHf (kJ/mol) E-H bond energy (kJ/mol) H-E-H bond angle (°)
NH3 -77.7 -33.35 -46.2 391 107
PH3 -133 -87.7 9.3 322 93.5
AsH3 -116.3 -55 172.2 247 92
SbH3 -88 -17.1 142.8 255 91.5
Figure 1: Plot of melting and boiling points of EH3 (E = N, P, As, and Sb) as a function of molecular mass.
Figure 1 (graphics1.jpg)

The E-H bond strengths decrease down the group and this correlates with the overall stability of each compound (Table 2). The H-E-H bond angles (Table 2) also decrease down the Group. The H-E-H bond angle is expected to be a tetrahedral ideal of 109.5°, but since lone pairs repel more than bonding pairs, the actual angle would be expected to be slightly smaller. Two possible explanations are possible for the difference between NH3 and the other hydrides.

  1. The N-H bond is short (1.015 Å) compared to the heavier analogs, and nitrogen is more electronegative than hydrogen, so the bonding pair will reside closer to the central atom and the bonding pairs will repel each other opening the H-N-H angle more than observed for PH3, etc.
  2. The accessibility of the 2s and 2p orbitals on nitrogen allows for hybridization and the orbitals associated with N-H bonding in NH3 are therefore close to sp3 in character, resulting in a close to tetrahedral geometry. In contrast, hybridization of the ns and np orbitals for P, As, etc., is less accessible, and as a consequence the orbitals associated with P-H bonding in PH3 are closer to p in character resulting in almost 90° H-P-H angle. The lower down the Group the central atom the less hybridization that occurs and the closer to pure p-character the orbitals on E associated with the E-H bond.

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