The chemical term “aromatic” is used to describe molecules with a planar, cyclic-shaped molecular orbital (region of space occupied by electrons) that contains 4n+2 (i.e. 2, 6, 10…) electrons. Benzene, the first molecule shown to possess such an electronic configuration, has a characteristically sweet smell. Early chemists presumably misattributed this aromatic property to the molecule’s electronic configuration, and the name stuck.
Today, our knowledge of aromatic molecules extends well beyond the realm of carbon-based organic compounds, with numerous examples of aromatic inorganic compounds having been discovered. However, most known aromatic compounds of chemical importance are π-aromatic –their cyclic aromatic orbital is formed from the longitudinal overlap of dumbbell-shaped atomic orbitals in a cyclic arrangement. σ-aromaticity, which describes a cyclic aromatic molecular orbital formed from the ends-on overlap of atomic orbitals, is much rarer – only certain all-metal clusters and interstellar hydrogenic species with little chemical utility are theorized to exhibit σ-aromaticity.
Last April, a team of chemists from the United States experimentally demonstrated, for the first time, σ-aromaticity through their work on zinc-platinum hydrides. The team generated a mixture of the bimetallic hydride ions PtZnH3–, PtZnH4–, and PtZnH5– in an undirected reaction. Mass spectrometric analysis of the ionic gaseous mixture showed an unusual abundance of the PtZnH5– species, and the photoelectric spectra of said species indicated that it had an exceptional bonding stability. Further calculations confirmed that this stability is indeed due to the σ-aromaticity that arose from the continuous overlap of the 5 hydrogen atoms’ σ-orbitals. Collateral to the aromaticity of the 5-hydrogen ring is the peculiar pentagonal PtH5 co-ordination geometry – platinum normally only co-ordinates to 4 other atoms in a square planar geometry. The “observer” Zn atom is positioned perpendicular to the plane of the ring to form a pentagonal pyramidal shape overall.
The team speculates that the aromatic metal hydride will be useful in the field of organic synthesis, where platinum-based complexes are often used as selective catalysts for certain reactions.
To find out more about this piece of research, please see the original article: http://pubs.acs.org/doi/abs/10.1021/jz500322n