The Cu(I)/Cu(II) redox couple is very amenable to 'tuning', it can be altered
over a wide range by suitable choice of complexing ligands and their geometry.
For example copper Schiff-base complexes, Cu(R-sal)2, can double
their redox potential by mere choice of R.
Cu(I) is a d10, closed shell ion, and since the most common coordination geometry is four, the preferred complex geometry will be tetrahedral. On the other hand Cu(II), with a d9 configuration, has square planar or octahedral geometry. The octahedral coordination usually has a strong Jahn-Teller distortion, lengthening the 5th and 6th bonds. Apart from 4- and 6-coordination both Cu(I) and Cu(II) coordination can adopt a wide range of geometries, both regular and irregular from 2 to 6, and Cu(II) can go up to 7 and 8 coordination. This flexibility is exploited by nature in the copper metalloenzymes.
Both geometry and the liganding atoms have a big influence. Thus a ligand set which imposes a tetrahedral geometry will stabilise Cu(I) with respect to Cu(II), and vice versa, a ligand set which imposes a square planar geometry will stabilise Cu(II) with respect to Cu(I). The effect can be seen vividly by comparison of acetonitrile and water as solvents for these ions. Cu(I) halides are readily soluble in acetonitrile, solvating as tetrahedral [Cu(MeCN)4]+, here, Cu(II) is a powerful oxidising agent. By contrast, in aqueous solution Cu(I) rapidly disproportionates to copper metal and hexacoordinate Cu(II) above about 10-2M. This is in part due also to the large solvation energy of Cu(II) in water, but illustrates the powerful influence of the coordination environment on copper chemistry. We can also regard Cu(I) as a soft acid ion in comparison to Cu(II), so that softer S- and N-base ligands are preferred over O-base ligands, and the opposite for Cu(II).
Thus it is possible to stabilise the Cu(I) system by immobilisation in a protein matrix, exclusion of dioxygen or complexation. Sulfur containing ligands are particularly effective at stabilising Cu(I) and the majority of Cu(I) thiolate species are stable. Cu(I) complexes are often two-coordinate with a linear arrangement of ligands, although three- , four- and five-coordinate complexes are also known.
Cu(II) thiolate compounds are found to be unstable with respect to disproportionation:
Once more, immobilization may yield kinetic stability in Cu(II) thiolate species, as occurs in the blue-copper family of electron-transport enzymes.