Inorganic imide

The inorganic imide is an inorganic chemical compound containing

• an anion with the chemical formula HN2−, in which nitrogen atom is covalently bonded to one hydrogen atom (as in lithium imide Li2NH and calcium imide CaNH). The other name of that anion is monohydrogen nitride.
• functional groups with the chemical formulas −NH− or =NH, in which nitrogen atom is also covalently bonded to one hydrogen atom, with two covalent single bonds or one covalent double bond from the nitrogen atom to other atoms, respectively (as in heptasulfur imide S7NH, sulfur diimide S(=NH)2 and nitroxyl O=NH).

Organic imides have the functional groups −NH− or =NH as well.

The imides are related to the inorganic amides, containing the H2N− anions, the nitrides, containing the N3− anions and the nitridohydrides or nitride hydrides, containing both nitride N3− and hydride H− anions.

In addition to solid state imides, molecular imides are also known in dilute gases, where their spectrum can be studied.

When covalently bound to a metal, an imide ligand produces a transition metal imido complex.

When the hydrogen of the imide group is substituted by an organic group, an organoimide results. Complexes of actinide and rare earth elements with organoimides are known.

Properties

Lithium imide undergoes a phase transition at 87 °C where it goes from an ordered to a more symmetric disordered state.

Structure

Many imides have a cubic rock salt structure, with the metal and nitrogen occupying the main positions. The position of the hydrogen atom is hard to determine, but is disordered.

Many of the heavy metal simple imide molecules are linear. This is due to the filled 2p orbital of nitrogen donating electrons to an empty d orbital on the metal.

Imides in coordination chemistry

In coordination chemistry transition metal imido complexes feature the NR2- ligand. They are similar to oxo ligands in some respects. In some the M-N-C angle is 180º but often the angle is decidedly bent. The parent imide (NH2-) is an intermediate in nitrogen fixation by synthetic catalysts.

Formation

Heating lithium amide with lithium hydride yields lithium imide and hydrogen gas. This reaction takes place as released ammonia reacts with lithium hydride.

Heating magnesium amide to about 400 °C yields magnesium imide with the loss of ammonia. Magnesium imide itself decomposes if heated between 455 and 490 °C.

Beryllium imide forms from beryllium amide when heated to 230 °C in a vacuum.

When strontium metal is heated with ammonia at 750 °C, the dark yellow strontium imide forms.

When barium vapour is heated with ammonia in an electrical discharge, the gaseous, molecular BaNH is formed. Molecules ScNH, YNH, and LaNH are also known.

Hydrogen storage

Inorganic imides are of interest because they can reversibly store hydrogen, which may be important for the hydrogen economy. For example, calcium imide can store 2.1% mass of hydrogen. Li2Ca(NH)2 reversibly stores hydrogen and release it at temperatures between 140 and 206 °C. It can reversibly hold 2.3% hydrogen. When hydrogen is added to the imide, amides and hydrides are produced. When imides are heated, they can yield hydridonitrides or nitrides, but these may not easily reabsorb hydrogen.

List

Ionic

Molecular

Molecular imines of other actinides called neptunimine and plutonimine have been postulated to exist in the gas phase or noble gas matrix.