In the animal world new species are discovered on a regular basis and the sighting of a rare species (the double tailed pigeon or purple Chichi-Sima tunicate) too may provoke media attention. So too in chemistry: David A. Lang et al. of Florida State University have stumbled on a rare sighting of the C34- anion of the carbide family and reports about it in the latest issue of JACS (DOI). The group's research actual target was a light-weight metal flux for use in alloys and as hydrogen storage material. A regular carbide is of the type MC2 such as calcium carbide with the carbon segment a deprotonated acetylide. The rare C3 fragment is called a sesquicarbide and can be found for example in Sc3C4. It can be regarded as a deprotonated allene.
The novel carbide (formally (Ca2+)2Li+(C3)4-(H-)) was synthesized by heating a mixture of calcium shot, lithium rod , carbon black and calcium hydride (10/10/6/1) at 1323 K. The product forms as 1 mm sized crystals in a Li/Ca melt that can be isolated by centrifugation. As expected in the solid state the C3 fragment is linear with C-C distance (132 pm) consistent with an allene. The calcium ions are present not only as an extension of the C3 unit but also perpendicular to it (pic). Lithium and hydrogen are present as alternating chains making it a double salt as in (Ca2C3)(LiH). Calcium and carbon alone will not form the C3 carbide. The new compound is oxidation prone and (violent) reaction with water yields allene itself.