Compounds of divalent copper

1.
Color

The colors of divalent copper compounds and complex ions are colorful, mainly including:

Anhydrous salt CuF ₂ white, CuCl ₂ brown-yellow, CuBr ₂ black, CuSO ₄ white, Cu(NO ₃ ) ₂ blue-green;

CuF ₂ ·2H ₂ O green, CuCl ₂ ·2H ₂ O green, CuSO ₄ ·5H ₂ O blue, CuAc ₂ ·H ₂ O green; Cu(NO ₃ ) ₂ ·3H ₂ O blue, Cu( NO ₃ ) ₂ ·6H ₂ O blue;

Oxide CuO black;

Hydroxide Cu(OH) ₂ light blue;

Complex ion [Cu(H ₂ O) ₄ ] ²⁺ blue, [CuCl ₄ ] ^2- yellow, [Cu(NH ₃ ) ₄ ] ²⁺ dark blue, [Cu(CN) ₄ ] ^2- yellow, [Cu( OH) ₄ ] ^2- Blue
.

2.
Copper hydroxide and copper oxide

A strong base is added to the soluble copper(II) salt solution to obtain copper hydroxide Cu(OH) ₂ precipitation
.

A Cu ₂ ⁺ + 2OH ^- = a Cu (OH) ₂

Cu(OH) _{2 is} amphoteric and partial alkali, which is partially soluble in strong alkali solution, but easily soluble in ammonia water
.

Cu(OH) _{2 has} poor thermal stability and is heated to about 80°C to be dehydrated and converted into copper oxide CuO
.

Cu(OH) ₂ =CuO+H ₂ O

CuO can also be made by decomposing certain oxo acid salts or heating copper powder in oxygen
.

CuO is an alkaline oxide, hardly soluble in water, but easily soluble in acid
.
CuO has oxidizing properties and can be reduced by reducing agents such as H2 at high temperatures

CuO+H ₂ =Cu+H ₂ O

CuO good thermal stability and not decompose when heated to the melting temperature (1227 deg.
] C), decomposition of a Cu ₂ required higher temperatures O

4CuO=2Cu ₂ O+O ₂

3.
Salt

The most important copper salt is copper sulfate pentahydrate CuSO ₄ ·5H ₂ O, blue, commonly known as bile alum
.
Four of the five H ₂ O coordinate to Cu ²⁺ , and the other H ₂ O forms a hydrogen bond with SO ₄ ^2-

CuSO ₄ ·5H ₂ O is commonly used in electroplating, pesticides (Bordeaux solution), algaecide for water purification, and is also a raw material for the preparation of other copper compounds
.
The anhydrous CuSO ₄ powder has strong water absorption and shows the characteristic blue color of hydrated copper ions after absorbing water.

Cupric halides include anhydrous salt of CuF ₂ , of CuCl ₂ and of CuBr ₂ , water and salt crystals of CuF ₂ · 2H ₂ O and of CuCl ₂ · 2H ₂ O
.
Anhydrous CuCl ₂ has an infinite chain structure

CuF _{2 is} slightly soluble in water, and CuCl ₂ and CuBr _{2 are} easily soluble in water
.
In a very concentrated CuCl ₂ aqueous solution, yellow [CuCl ₄ ] ^{2- is formed} ; while the dilute CuCl ₂ solution is blue, and the solution is mainly [Cu(H ₂ O) ₄ ] ^2-

Hydrolysis reaction occurs when CuCl ₂ ·2H ₂ O is heated

2CuCl ₂ ·2H ₂ O=Cu(OH) ₂ ·CuCl ₂ +2HCl+2H ₂ O

Therefore , when anhydrous CuCl ₂ is prepared by the dehydration method , it must be carried out in an HCl atmosphere
.
Anhydrous CuCl ₂ is further decomposed into CuCl by heating at high temperature

2CuCl ₂ =2CuCl+Cl ₂

Anhydrous copper nitrate Cu(NO ₃ ) ₂ is easy to sublime under vacuum, and it is monomer in gaseous state
.

Copper sulfide CuS is hard to dissolve in water, but can be dissolved in concentrated nitric acid or cyanide solution
.

4.
Coordination Compound

Cu ²⁺ forms a four-coordinated square or planar quadrilateral complex ion with a single-base ligand, such as [CuCl ₄ ] ^2- , [Cu(H ₂ O) ₄ ] ²⁺ , [Cu(NH ₃ ) ₄ ] ²⁺ , [a Cu (the CN) _.
4 ] ^2- and the like
.
Cu ²⁺ may also have six-coordinate (elongated octahedron) and five-coordinate (tetragonal pyramid or triangular bipyramid).

Cu ²⁺ can also form stable coordination compounds with some organic ligands (such as ethylenediamine, etc.
)
.

5.
Reducibility

Cu(Ⅲ) compounds are rare due to poor stability
.
K ₃ CuF ₆ can be obtained by co-heating copper chloride , metallic potassium and elemental fluorine

5K+CuCl ₂ +3F ₂ =K ₃ CuF ₆ +2KCl

The following reaction can also produce Cu(Ⅲ) compounds

2CuO+2KO ₂ =KCuO ₂ +O ₂