Although they are sometimes referred to as nitrogen triiodide (1, 4), the black, explosive crystals formed by reaction of iodine with concentrated aqueous ammonia are not NI₃, but NI₃·NH₃. This material does not contain discrete NI₃ molecules but it has a polymeric structure with tetrahedral NI₄ units sharing two corners to give zigzag chains. In addition, one NH₃ molecule is attached by a weaker bond to alternate nonbridging iodine atoms along the chain (5).

Many textbooks, including some published in recent years (6-8), state that unsolvated NI₃ has not been isolated. Nevertheless, it was prepared in 1990 by reaction of boron nitride with IF in CFCl₃, and characterized by Raman and ¹⁵N NMR spectroscopies (9), as acknowledged in Cotton and Wilkinson's textbook (10).

Some simple calculations help students to understand that, while NF₃ is thermodynamically stable, the NX₃ (X = Cl, Br, I) are explosive dangerous materials. By using the data and equations reported in ref 11, it can be seen that reaction 1 is exothermic when X = F, but highly endothermic when X = Cl, Br, I.

¹/₂N₂(g) + ³/₂X₂(g) NX₃(g)     (1)

With the data in ref 11 and the sublimation enthalpy of iodine (62 kJ/mol), the enthalpy of formation of NI₃(g) can be calculated to be +272 kJ/mol, in good agreement with the published value of +287 ± 23 kJ/mol (12). The gaseous compound is greatly stabilized by formation of the crystalline adduct with ammonia, since the enthalpy of formation of NI₃·NH₃(c) is +146 ± 6 kJ/mol (12). Still, NI₃·NH₃ explodes according to eq 2 and the enthalpies of formation of NH₃(g) (-46 kJ/mol) and I₂(g) (+62 kJ/mol) lead to a calculated enthalpy change for reaction 2 of -99 kJ/mol.

NI₃·NH₃(c) NH₃(g) + ¹/₂N₂(g) + ³/₂I₂(g)     (2)

Students may ask why chemical reactions can lead to the formation of very unstable compounds such as NI₃ and NI₃·NH₃. The answer is that very stable compounds are also formed, thus providing the driving force for the reactions. For example, the reaction of iodine crystals with aqueous ammonia is represented by eq 3:

3I₂(c) + 5NH₃(aq) NI₃·NH₃(c) + 3NH₄I(aq)     (3)

Although the enthalpy of formation of NI₃·NH₃(c) is +146 kJ/mol, the enthalpies of formation of NH₃(aq) (-80 kJ/mol) and NH₄I(aq) (-188 kJ/mol), lead to an enthalpy change for reaction 3 of -18 kJ/mol, the stability of NH₄I(aq) being a critical factor for the reaction to take place. Also, the formation of gaseous NI₃ according to eq 4 is a thermodynamically favored process because of the high stability of BF₃. Indeed, the enthalpies of formation of BN(c) (-254 kJ/mol), IF(g) (-96 kJ/mol), BF₃(g) (-1136 kJ/mol), and NI₃(g) (+287 kJ/mol) lead to a calculated enthalpy change for reaction 4 of -307 kJ/mol.

BN(c) + 3IF(g) BF₃(g) + NI₃(g)     (4)

Literature Cited

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3. Jacobsen J. J.; Moore, J. W. Chemistry Comes Alive! Vol. 3; J. Chem. Educ. Software 2000, SP 23.
4. Hambly, G. H.; Peters, R. J. Chem. Educ. 1993, 70, 943.
5. Jander, J. Adv. Inorg. Chem. Radiochem. 1976, 19, 1.
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10. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; p 338.
11. Kildahl, N. K. J. Chem. Educ. 1995, 72, 423.
12. Davies, R. H.; Finch, A.; Gates, P. N. J. Chem. Soc., Chem. Commun. 1989, 1461.