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 deca- coordinate structures with boron atoms serving as the ligand. Recently, such planar hexa- and penta-coordinate transition metals as FeBi5¨C have been predicted. Nevertheless, none of transition metals centered in a seven-member or greater boron ring has heretofore been investigated.

Based on chemical bond theory, a series of structures were designed containing planar octa- and ennea- coordinate cobalt, iron and nickel atoms. Density functional methods and a gaussian basis set with polarization and dispersion were applied to optimize the molecules. The molecules, singlet 1FeB8-2, multiplet kFeB9n (n = -1, k = 1; n = 0, k = 2), singlet 1CoB8n(n = -1, +1, +3), multiplet kCoB9n (n = +1, k = 2; n = 0, k = 1) and singlet 1NiB9+1, were found to be especially stable, suitable for experimental verification.

According to Luo Qiong, ¡°The stable structures containing the planar octa- and ennea-coordinate cobalt, iron and nickel were first theoretically considered in order to identify potential candidates for experimental synthesis. This study contributed to understanding of planar hyper-coordinate transition metals (PHTMs).¡±

To gain a better understanding concerning which electronic factors contribute to the stabilization of these planar molecules, the nature of the chemical bonding structures, involving the HOMO orbital, the HOMO-LUMO gap, and the aromaticity, was evaluated. The report notes that the stability of these structures is a result of the contribution from two kinds of orbitals, an extensively delocalized ¦Ò orbital on the molecular plane and a delocalized ¦Ð orbital vertical to the plane. These species are aromatic with six delocalized p electrons, which is consistent with 4n+2 p electrons aromatic rules.

If one of these planar hyper-coordinate cobalt, iron and nickel molecules is experimentally confirmed, it might have a significant impact on chemistry theory and relevant applications.

Source: Science in China Press