Page 416 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

the (II) and (III) oxidation states (Cotton and Wilkinson 1967). Even in the (I) oxidation state there
are many substances, such as benzotriazole or the olefins, that can complex with copper (I).
The standard electrode potentials for copper are
Cu 2 + + 2e = Cu +0.34V
_
Cu + + e" = Cu +0.54V

Cu 2 + - e" = Cu + +0.17V

The equilibrium constant between Cu(I) and Cu(II) is

2Cu + = Cu 2 + + C u K = a C u 2 + / ( a C u ) 2 1 X 10 at 25 °C
6
+
The constant for this reaction, K, therefore favors Cu(II) over Cu(I) under equilibrium
conditions.
Although copper, silver, and gold are all face-centered cubic metals, there is only moderate
similarity in their chemistry. In terms of metallic properties, however, they are all ductile, mal­
leable, and good conductors, and all have similar casting and working properties.

Characteristics of The electronic structure of copper (I) compounds, or cuprous
copper compounds compounds, is d . These compounds are diamagnetic and
3
1 0
tend to be colorless or very pale, such as nantokite, cuprous
chloride, which is colorless to pale green. The exception is when color results from the anion or
charge-transfer bands, as in the case of cuprite, Cu 2 0. Most cuprous compounds can be oxi­
dized easily to cupric ones, but oxidation to tervalent copper (III) compounds is difficult; they
are relatively rare, existing only in the laboratory.
The copper (II) compounds, or cupric compounds, make up the most important copper
group and form a wide range of minerals. The Cu 2 + ion, which has a d 9 configuration, always
3
exhibits Jahn-Teller effects. Consequently, instead of regular octahedral coordination, which is
characteristic of many transition metal complexes, appreciable distortions of the regular octa­
hedral coordination occur. Depending on the ligand, a wide range of geometries are found:
tetrahedral, trigonal bipyramidal, square pyramidal, square, and distorted octahedral. In many
compounds, the distortions are so great that the coordination is practically square. An example
is chalcanthite, CuS0 4 -5H 2 0, which has four oxygen atoms from the water molecules in one
plane, forming the corners of a square, and oxygen atoms from sulfate groups occupying each
axial position. The additional water molecule is hydrogen bonded between a second sulfate oxy­
gen and a bound water molecule in the plane. Some copper compounds, such as neutral verdi­
gris, are dimeric structures, although they may be partially dissociated in aqueous solution. The
strength of the copper-to-copper bond in these carboxylates is only about 1 kcal/mol (4 kj/mol).
Thus the bond is weak, and the distance between the copper atoms is long compared with the

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