In inorganic chemistry, alkali metal cuprates(III) refers to a family of copper salts of the form , where M is an alkali metal. They are cuprate salts and coordination polymers. Their primary interest is as simple models of the polymeric cuprate(III) sheet anion, notionally , which also appears in cuprate superconductors.
Cuprates(III) are very strong oxidants. They are unstable in water, typically oxidizing the medium.
One of the simplest oxide-based cuprates is potassium cuprate(III) . This species can be viewed as the salt of the polyanion . As such the material is classified as an oxide cuprate. This dark blue diamagnetic solid is produced by heating potassium peroxide and copper(II) oxide in an atmosphere of oxygen:
was discovered first in 1952 by V. K. Wahl and W. Klemm, they synthesized this compound by heating copper(II) oxide and potassium superoxide in an atmosphere of oxygen.
It can also be synthesized by heating potassium superoxide and copper powder:
reacts with the air fairly slowly. It starts to decompose at and its color changes from blue to pale green at . Its melting point is .
In fact, is a non-stoichiometric compound, so the more exact formula is and x is very close to 2. This causes the formation of defects in the crystal structure, and this leads to the tendency of this compound to be reduced.
Sodium cuprate(III) can be produced by using hypochlorites or hypobromites to oxidize copper hydroxide under alkaline and low temperature conditions.
Sodium cuprate(III) is reddish-brown, but turns black gradually as it decomposes to copper(II) oxide. In order to prevent decomposition, it must be prepared at low temperature in the absence of light.
Other cuprates(III) of alkali metals are known; the structures of (potassium cuprate(III)), (rubidium cuprate(III)) and (caesium cuprate(III)) have been determined as well.
(blue-black) and (black) can be prepared by reaction of rubidium oxide and caesium oxide with copper(II) oxide powders, at and in oxygen atmosphere, respectively. They both reacts with air rapidly, unlike .