Seeing an orbitalA paper published in the research journal Nature in 1999 (Direct observation of d-orbital holes and Cu-Cu bonding in Cu2O, J. M. Zuo, M. Kim, M. O'Keeffe, and J. C. H. Spence, Nature, 401, 49, 1999) caused great excitement in the chemistry world. Through a series of very careful experiments using modern X-ray and electron diffraction methods, these authors measured the electron distribution around Cu atoms in solid Cu2O, copper(I) oxide. They found direct evidence for a charge distribution around each Cu atom that should look familiar to you:
This picture, adapted from their artical, shows electron density as both contours of constant density (the dashed lines) and shades of color. Look at the dark purple region: it's a dz2 orbital! Interestingly, in this compound, they found that this orbital was empty. The way they analyzed their data let them highlight regions in space that had less electron density that a spherical Cu+ ion would have. (Remember: Cu has the electron configuration [Ar] 3d10 4s1 so that bare Cu+ has the spherical [Ar] 3d10 configuration.) They were able to interpret this loss of electron density in a way that showed Cu+ in the solid was non-spherical. Why? Because the electrons that should have been in this orbital were off dancing with another Cu+, forming Cu–Cu bonds throughout the solid!  |
