Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 71
Copper is usually found in Nature in association with sulfur. Pure copper
metal is generally produced from a multistage process, beginning with the
mining and concentrating of low-grade ores containing copper sulfide minerals,
and followed by smelting and electrolytic refining to produce a pure
copper cathode. An increasing share of this metal is produced from acid
leaching of oxidized ores. Because of its properties which include high ductility,
malleability, thermal and electrical conductivity, and resistance to corrosion,
copper has become a major industrial metal, ranking third after iron
and aluminum in terms of quantities consumed. Electrical uses of copper,
including power transmission and generation, building wiring, telecommunication,
and electronic products, account for about three quarters of total
employment.Today,copperby-productsfrommanufacturingandobsolete
copper products are readily recycled and contribute significantly to supply.
This is becoming a necessity due to the increasing difficulty of production to
meet current world demand which has led to a quintuplication of the copper
price during the last seven years, rising from $0.60/pound in June 1999
to $3.75/pound in May 2006. In 2005 14.9 million tons of copper were mined
around the world; the global world reserves, economically recoverable with
current technologies, amount to 470 million tons [1]. It has been recently
estimated that ca. 25% of the copper stock initially available in the lithosphere
has been already placed in use or in wastes from which it will probably
never be recovered. This poses concern about the sustainability of current
consumption trends of such a valuable commodity in the mid-long term [2].
1.2
Chemical Properties, Coordination Geometries, Excited States—Cu(I) vs. Cu(II)
Copper is a transition element belonging to the same group of the periodic
table as gold and silver, these elements are sometimes referred to as the coinage
metals in recognition of their historically widespread use in stamping coins.
Copper has a single s electron outside the filled 3d shell but its properties have
essentially nothing in common with alkali metals except for the possibility of
assuming the +1 oxidation state. The filled d shell is not very effective in shielding
the s electron from the nuclear charge, so the first ionization enthalpy of Cu
is higher than that of the alkali metals. Since the electrons of the d shell are also
involved in metallic bonding, the heat of sublimation and the melting point of
Cu are also much higher than those of the alkalis. The above factors, taken together,
are responsible for the noble character of copper. Indeed copper is the
only industrial pure metal, used on a massive scale, exhibiting a positive electrochemical
potential: for this reason it is not corroded by acids, unless they
are strongly oxidizing like HNO3 and H2SO4.
Copper in solution has two common oxidation states: + 1 and + 2. Because
of their intrinsically superior photochemical and photophysical properties
(vide infra), in this review our attention is focused on Cu(I) complexes, which