Copper is situated in the first row of the transition elements and has an electron configuration of [Ar]4s13d10. The chemistry of copper is dominated by two oxidation states, I and II, although Cu(III) complexes have been reported.
Copper (I) has the electron configuration [Ar]3d10, so its complexes tend to be colorless and diamagnetic. It prefers "soft" Lewis bases such as thioethers, phos-phines, and nitriles, and usually forms tetradentate complexes that adopt a tetrahedral environment. If Cu(I) is bound by weakly coordinating ligands, it dis-proportionates in solution to give Cu(0) and Cu(II).
Cu(II) has an electron configuration of [Ar]3d9, so all mononuclear Cu(II) complexes are paramagnetic. Cu(II) can be termed an "intermediate" Lewis acid and as such is bound most strongly by nitrogen- and sulfur-containing ligands. The co-ordination number can vary from four to six, with tetradentate complexes preferring a square-pyramidal arrangement and hexa-dentate complexes adopting a distorted octahedral environment. The distorted octahedral environment is caused by the partially filled d-orbital that causes tetragonal elongation along the z-axis.
Cu(III) complexes are rare and require strong n-donating ligands for stability. The copper in these species have a [Ar]3d8 electronic configuration similar to that for Ni(II) and form predominately square planar complexes. Cu(III) complexes are powerful oxidants and in solution require a high pH to remain stable; because of this they cannot be utilized in biological systems.
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