Rule #1: No more than four bonds to carbon.
It’s the first important rule you learn in sophomore organic chemistry. When you write an exam [1], putting five bonds to carbon (the infamous “Texas carbon”) is akin to going to the Vatican and telling everyone Jesus was black. People are going to yell at you and tell you you’re wrong, they instruct you to go to the Sistine Chapel and look up, and you might even get excommunicated, but you still know better. Well, this is one of those “Jesus is black” papers. [2] It’s a step up from the texas carbon (which have already been characterized)- it’s a Canada carbon. [3]
Well, kind of. Yamamoto et al. describe a molecule with a “hexacoordinate” carbon in it, though it’s not six covalent bonds like the kind sophomore orgo drills you on- they’re more like dative bonds, more akin to inorganic chemistry. The molecule is a weird bis-anthracene with an allene bridge, which looks not unlike Feringa’s molecular motors. The methylated version, shown below, requires highly unreactive carborane counterions for x-ray analysis. The carbon in the middle of the allene bridge, they claim, is hexacoordinate, as the four peripheral oxygens donate a lone pair into the carbon’s HOMOs.

Using x-ray crystallography, they found that the allene carbon-oxygen bond lengths (around 2.6 Å) are longer than a covalent C-O bond but shorter than the sum of the van der Waals radii, implying that they might be bonding. High-resolution x-ray anaylsis (using a synchotron!) showed that there were bond paths between the carbon and oxygen, although the interactions were pretty weak. DFT shows that the oxygen is likely donating electron density into the π* orbital of the carbon, not unlike a charge transfer complex.
So… not exactly a Canada carbon, but a pretty interesting complex, nonetheless. It’s always fun to read papers that, had you seen them when you were in sophomore orgo, would have made your head asplode.
[1] Americans say “take exams” but I like “write exams” more.
[2] Politically correct? moi?
[3] Everything might be bigger in Texas, but in Canada everything’s so big you have to drink to forget about it.
Yamaguchi, T., Yamamoto, Y., Kinoshita, D., Akiba, K., Zhang, Y., Reed, C.A., Hashizume, D., Iwasaki, F. (2008). Synthesis and Structure of a Hexacoordinate Carbon Compound. Journal of the American Chemical Society DOI: 10.1021/ja710423d