Copper(II) nitrate

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Copper(II) nitrate

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Solubility in ethanol	hydrates very soluble
Solubility in ammonia	hydrates very soluble
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Copper(II) nitrate describes any member of the family of inorganic compounds with the formula Template:Chem2. The hydrates are hygroscopic blue solids. Anhydrous copper nitrate forms blue-green crystals and sublimes in a vacuum at . Common hydrates are the hemipentahydrate and trihydrate.

Synthesis and reactions

Hydrated copper(II) nitrate

Hydrated copper nitrate is prepared by treating copper metal or its oxide with nitric acid:

Template:Chem2

The same salts can be prepared treating copper metal with an aqueous solution of silver nitrate. That reaction illustrates the ability of copper metal to reduce silver ions.

In aqueous solution, the hydrates exist as the aqua complex Template:Chem2. Such complexes are highly labile and subject to rapid ligand exchange due to the d⁹ electronic configuration of copper(II).

Attempted dehydration of any of the hydrated copper(II) nitrates by heating affords the oxides, not Template:Chem2. At the hydrates convert to "basic copper nitrate", Template:Chem2, which converts to Template:Chem2 at . Exploiting this reactivity, copper nitrate can be used to generate nitric acid by heating it until decomposition and passing the fumes directly into water. This method is similar to the last step in the Ostwald process. The equations are as follows:

Template:Chem2

Template:Chem2

Treatment of copper(II) nitrate solutions with triphenylphosphine, triphenylarsine, and triphenylstibine gives the corresponding copper(I) complexes Template:Chem2 (E = P, As, Sb). The group V ligand is oxidized to the oxide.

Anhydrous copper(II) nitrate

Anhydrous Template:Chem2 is one of the few anhydrous transition metal nitrates. It cannot be prepared by reactions containing or producing water. Instead, anhydrous Template:Chem2 forms when copper metal is treated with dinitrogen tetroxide:

Template:Chem2

Structure

Anhydrous copper(II) nitrate

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Structure of anhydrous copper(II) nitrate in the gas phase.

Two polymorphs of anhydrous copper(II) nitrate, α and β, are known. Both polymorphs are three-dimensional coordination polymer networks with infinite chains of copper(II) centers and nitrate groups. The α form has only one Cu environment, with [4+1] coordination, but the β form has two different copper centers, one with [4+1] and one that is square planar.

The nitromethane solvate also features "[4+1] coordination", with four short Cu-O bonds of approximately 200 pm and one longer bond at 240 pm.

Heating solid anhydrous copper(II) nitrate under a vacuum to leads to sublimation and cracking to give a vapour of monomeric copper(II) nitrate molecules. In the vapour phase, the molecule features two bidentate nitrate ligands.

Hydrated copper(II) nitrate

Five hydrates have been reported: the monohydrate (Template:Chem2), the sesquihydrate (Template:Chem2), the hemipentahydrate (Template:Chem2), a trihydrate (Template:Chem2), and a hexahydrate (Template:Chem2.

The crystal structure of the hexahydrate appeared to show six almost equal Template:Chem2 distances, not revealing the usual effect of a Jahn-Teller distortion that is otherwise characteristic of octahedral Cu(II) complexes. This non-effect was attributed to the strong hydrogen bonding that limits the elasticity of the Template:Chem2 bonds but it is probably due to nickel being misidentified as copper in the refinement.

Applications

Copper(II) nitrate finds a variety of applications, the main one being its conversion to copper(II) oxide, which is used as catalyst for a variety of processes in organic chemistry. Its solutions are used in textiles and polishing agents for other metals. It is often used in school laboratories to demonstrate chemical voltaic cell reactions. It is a component in some ceramic glazes and metal patinas.

Organic synthesis

Copper nitrate, in combination with acetic anhydride, is an effective reagent for nitration of aromatic compounds, known as the Menke nitration.

Hydrated copper nitrate adsorbed onto clay affords a reagent called "Claycop". The resulting blue-colored clay is used as a slurry, for example for the oxidation of thiols to disulfides. Claycop is also used to convert dithioacetals to carbonyls. A related reagent based on montmorillonite has proven useful for the nitration of aromatic compounds.

Electrowinning

Copper(II) nitrate may also be used for copper electrowinning on small scale with a ammonia (Template:Chem2) as a byproduct.

Naturally occurring copper nitrates

No mineral of the ideal Template:Chem2 formula, or the hydrates, are known.

Likasite, Template:Chem2 and buttgenbachite, Template:Chem2 are related minerals. Natural basic copper nitrates include the rare minerals gerhardtite and rouaite, both being polymorphs of Template:Chem2. A much more complex, basic, hydrated and chloride-bearing natural salt is buttgenbachite.