About 125 years ago, chemists discovered that carbon, life's central element, normally bonds with four other atoms or groups in a tetrahedral configuration. The low energy requirements of this three dimensional structure make it very stable, and so for decades it was assumed that carbon compounds could only occur in this arrangement. In the early 1970s, however, researchers determined that carbon molecules could in fact exist in flat configurations. Since then, planar arrangements of carbon molecules and other five-atom molecules have been created in the laboratory. Now new research, published today in the journal Science, reveals that another form of flat carbon might exist--one in which carbon has six neighbors instead of four.
To investigate the possibility of such planar "hexacoordinate" carbon molecules, Paul von Ragu Schleyer and Kai Exner of the University of Georgia used computational techniques to design various compounds involving carbon and boron, with carbon in the center of six-atom rings. After fiddling with bond lengths, the team found three arrangements that worked. "We have shown that planar configurations with more groups than anyone had imagined before can exist," Schleyer reports. "What makes this discovery remarkable is that it violates two basic tenets of carbon chemistry--that carbon should have four bonds, or neighbors, not six, and that molecules should be in 3-D arrangements--simultaneously." Although such configurations are only theoretical at this point, the authors estimate that experimental observation should be possible.