Entries in FeMoCo (1)


Carbon bond formation may be more flexible than we think!  

Are you an organic chemist and cringe when you see a carbon with five bonds drawn in your student’s homework? Cringe no more, it may not be as wrong as you think!

Recently, Malischewski and Seppelt were able to isolate the hexamethylbenzene dication under superacidic conditions and even obtain a crystal structure [1]. One of the carbon atoms appears to be bound to six (!) other carbon atoms, which is definitely more than the four bonds that we’re used to drawing in introductory organic chemistry. Now, before you get nervous - the actual bond orders are significantly lower than one, so if you sum up all six, the magic limit of four is not exceeded, the octet isn’t violated and generations of organic chemists can continue to sleep in peace. To be fair, a similar effect had previously been described in the context of the non-classical norbornyl carbocation [2]. The concept of carbon with more than four binding partners isn’t as artificially constructed as one might think, it actually exists in nature and has a profound impact on all of us. FeMoCo, an enzyme cofactor essential to nitrogen fixation, has a hexacoordinated C4- center [3]. Now that’s unique!

Bottom line, as long as the bonds aren’t the classical 2-center-2-electron bonds, carbon can be more flexible than we typically give it credit for... you probably still shouldn’t draw more than four bonds per carbon in your average homework assignment though.



[1]Angewandte Chemie International Edition 2016, 56, 368-370

[2] Science 2013, 341, 62-64

[3] Science 2011, 334, 974-997