A striking feature of metal oxide chemistry is the unusual electronic and chemical behaviour of Cu(I) and Ag(I): a case in point is that detailed understanding of Cu-O bonding is essential to the theory of high-temperature copper oxide superconductors. Both cations are usually coordinated in a linear fashion to two oxygens, particularly for Cu(I). In many compounds, the Cu(I) and Ag(I) cations also adopt close-packed (and related) configurations with short metal-metal distances that are strongly suggestive of the occurrence of metal-metal bonding despite their formal nd10 configuration. Such observations have been explained by invoking the participation in bonding of electronic orbitals of higher principal quantum number - that is, (n + 1)s and (n + 1)p - accompanied by the creation of d-orbital holes on the metal ion. To test this hypothesis, we have used a recently developed method of quantitative convergent-beam electron diffraction combined with X-ray diffraction to map the charge-density distribution in the simple oxide Cu2O, the results of which we then compare with electronic-structure calculations. We are able to image directly the d holes on the copper atoms, and also demonstrate the existence of Cu-Cu bonding in this compound.
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