Credit: © 2009 Wiley

The group-14 elements silicon, germanium, tin and lead form anionic homometallic clusters that have been studied since the nineteenth century. They also form clusters with transition metals, in a range of sizes up to 48 atoms. These, however, have so far been limited to forming around late transition metals such as nickel, palladium and platinum.

Now, Thomas Fässler and colleagues from the Technical University of Munich have prepared1 a compound that contains a cobalt-centred ten-germanium cluster. The compound also contains a potassium-cryptand ion and germanium-cyclooctadiene species, so the cluster has been assigned a triple-negative charge. Its structure is significantly different from the expected bicapped square antiprism, because the cobalt atom is sandwiched between two pentagons of germanium atoms. This pentagonal prism is close to geometric perfection, with all edges nearly the same length and the internal angles close to 108°.

Fässler and co-workers used density functional theory to examine the structure of the cobalt cluster and other isoelectronic analogues containing nickel, copper and zinc, as well as the empty germanium cage. Interestingly, only the cobalt cluster is predicted to form this pentagonal prismatic form, because the bicapped square antiprism structure is predicted to be more stable for all the others.