Nitrogen Trichloride and the Oxides of Nitrogen

Nitrogen trichloride, $\text{NCl}_3$, is prepared by passing chlorine gas over ammonia in

large excess, or by the chlorination of an ammonium salt solution; if the ammonia is not kept in

large excess the product can further react/decompose violently, so the preparation requires

careful control of stoichiometry. Structurally, $\text{NCl}_3$ closely resembles $\text{NH}_3$:

nitrogen is $sp^3$ hybridised, three of its hybrid orbitals form $\sigma$ bonds to the three

chlorine atoms, and the fourth holds the lone pair, giving the same trigonal pyramidal shape,

with a $\text{Cl-N-Cl}$ bond angle of roughly $107^\circ$, close to ammonia's own bond angle.

Unlike ammonia, however, $\text{NCl}_3$ is a pale-yellow, oily, highly unstable and dangerously

explosive liquid, decomposing readily (sometimes with detonation on shock or slight heating)

to nitrogen gas and chlorine, and it is also readily hydrolysed by water to give ammonia and

hypochlorous acid:

This instability again reflects nitrogen's small size and its inability, unlike phosphorus, to

use $d$ orbitals to stabilise an expanded or unusual bonding arrangement -- the N-Cl bonds in

$\text{NCl}_3$ are comparatively weak and easily broken.

Nitrogen also forms a remarkable series of oxides, one for nearly every whole-number

oxidation state from $+1$ to $+5$, and this chapter covers their structures only (not detailed

preparation/reactions). Nitrous oxide, $\text{N}_2\text{O}$ ($+1$, "laughing gas"), is a

linear molecule, $\text{N=N=O}$ (isoelectronic with $\text{CO}_2$), with nitrogen sp hybridised.

Nitric oxide, $\text{NO}$ ($+2$), is a small, linear diatomic molecule that is

paramagnetic, since molecular orbital theory places its one unpaired electron in an

antibonding $\pi^*$ orbital; on exposure to air it is readily oxidised to brown $\text{NO}_2$

fumes. Dinitrogen trioxide, $\text{N}_2\text{O}_3$ ($+3$, the formal anhydride of nitrous

acid $\text{HNO}_2$), is a planar molecule built from an $\text{NO}$ unit and an $\text{NO}_2$

unit joined through a central N-N bond. Nitrogen dioxide, $\text{NO}_2$ ($+4$), is a bent

("V-shaped") molecule with a bond angle of about $134^\circ$ (wider than a typical bent triatomic

because of an unpaired, odd electron on nitrogen rather than a full lone pair), and is itself

paramagnetic; it exists in equilibrium with its colourless dimer $\text{N}_2\text{O}_4$, in which …