Direct observation of d-orbital holes and Cu–Cu bonding in Cu2O

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 1 , 2 despite their formal nd 10 configuration. Such observations have been explained 3 , 4 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 5 combined with X-ray diffraction to map the charge-density distribution in the simple oxide Cu 2 O, 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.