Climbers scale Mt. Everest, because its there, and chemists have their own version of this. Ever since G. N. Lewis introduced the concept of the electron-pair bond in 1916, the idea of a bond as having a formal bond-order has been seen as a useful way of thinking about molecules. The initial menagerie of single, double and triple formal bond orders (with a few half sizes) was extended in the 1960s to four, and in 2005 to five. Since then, something of a race has developed to produce the compound with the shortest quintuple bond. One of the candidates for this honour is shown below[1] which is a crystalline species (a few diatomics which exist in the gas phase are also candidates; for other reviews of the topic see [2],[3] and [2].
References
- C. Hsu, J. Yu, C. Yen, G. Lee, Y. Wang, and Y. Tsai, "Quintuply‐Bonded Dichromium(I) Complexes Featuring Metal–Metal Bond Lengths of 1.74 Å", Angewandte Chemie International Edition, vol. 47, pp. 9933-9936, 2008. https://doi.org/10.1002/anie.200803859
- Y. Tsai, and C. Chang, "Recent Progress in the Chemistry of Quintuple Bonds", Chemistry Letters, vol. 38, pp. 1122-1129, 2009. https://doi.org/10.1246/cl.2009.1122
- Y. Tsai, H. Chen, C. Chang, J.K. Yu, G. Lee, Y. Wang, and T. Kuo, "Journey from Mo−Mo Quadruple Bonds to Quintuple Bonds", Journal of the American Chemical Society, vol. 131, pp. 12534-12535, 2009. https://doi.org/10.1021/ja905035f