Do the hyper-coordinate planar transition metal atoms exist?
A study reported in Vol 51, Issue 7 (July, 2008) of Science in China Series B: Chemistry has shown that wheel-shaped structures with octa- and enneacoordinate planar cobalt, iron and nickel centered in perfect octagonal and enneagonal boron rings, are stable on corresponding potential hyper-surfaces. This suggests that the central element bonding capacities have not been exhausted. In coordination compounds, the ligands have long been assumed to bond to coordination center elements at various orientations in three-dimensional space. For example, the four H atoms in methane molecules exist in a tetrahedral configuration. Since the possibility of tetracoordinate planar carbon (TPC) was first proposed by Hoffmann et al., numerous studies on hypercoordinate planar structures have been carried out. The most common ligand is a boron atom. Besides carbon, other primary group elements, such as Si, Ge, Sn, and Pb, were found to be capable of forming planar hexa-, planar hepta- and even planar d
