As I have noted elsewhere, Gilbert N. Lewis wrote a famous paper entitled “the atom and the molecule“, the centenary of which is coming up. In a short and rarely commented upon remark, he speculates about the shared electron pair structure of acetylene, R-X≡X-R (R=H, X=C). It could, he suggests, take up three forms. H-C:::C-H and two more which I show as he drew them. The first of these would now be called a bis-carbene and the second a biradical.
- G.N. Lewis, "THE ATOM AND THE MOLECULE.", J. Am. Chem. Soc., vol. 38, pp. 762-785, 1916. http://dx.doi.org/10.1021/ja02261a002