Compounds of Univalent Copper

In water, the cuprous ion, Cu+, may not exist in appreciable quantities, for it disproportionates (dismutates) into the cupric ion, Cu2+, and copper metal. Certain very slightly dissociated complexes of univalent copper (for example, Cu(CN)J3 and CuClJ ) are stable in aqueous solutions and relatively insoluble cuprous compounds (for example, CuCl, and CU2O) may survive in the presence of water if strong oxidizing agents are not also present. The iodide, Cul, and sulfide, Cu2S, are particularly stable. Aside from the instability of the hydrated Cu+ ion, the chemistry of univalent copper is quite similar to that of univalent silver. 

Reduction of the copper(II) halides to copper(I) halides may be carried out using sulfite or copper metal. If the reduction with copper metal is carried out in concentrated HC1 solution, the solution takes on an intermediate black color before fading to the colorless CuCl ion. The black color is probably due to dimeric or polymeric ions having copper in both the +1 and +2 states, for example, Cl—Cu—Cl—CuC12(H20) . The reduction of divalent copper by cyanide to form the cyano complex, Cu(CN)73, was mentioned in Chapter 10, and the reaction between cupric and iodide ions is similar to this, releasing iodine and forming the very insoluble copper iodide  

This reaction may be used to estimate Cu2+ in solution for, it may be remembered, free iodine (or, more conveniently) the tri-iodide ion, I7, may be easily titrated with sodium thiosulfate. Cuprous oxide, Cu20 

The cupric ion has an incomplete 3d subshell of nine electrons in common with nearly all ions with partially filled d subshells, its compounds are colored (the two conspicuous exceptions being the anhydrous fluoride and sulfate, CuF2 and C11SO4). The most deeply colored compounds or complexes are those in which the bonds from Cu24 are most nearly covalent thus, the Cu—O bonds (to oxygens of the sulfate) in anhydrous copper(II) sulfate (white) are more ionic than the Cu—O bonds (to oxygens of the water) in CuSOr5HsO (blue) which are, in turn, more ionic than the Cu—Br bonds in cupric bromide (brown). 

Although the blue hydrates of copper(II) sulfate, copper(II) nitrate, and eopper(II) acetate are common reagents, the anhydrous bromide and chloride, both of which are deep brown, are less familiar. The latter two 

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