Henry Rzepa's Blog

The structure of ferrocene was famously analysed by Woodward and Wilkinson in 1952[1],[2], symmetrically straddled in history by Pauling (1951) and Watson and Crick (1953). Quite a trio of Nobel-prize winning molecular structural analyses, all based on a large dose of intuition. The structures of both proteins and DNA succumbed to models built from simple Lewis-type molecules with covalent (and hydrogen) bonds; ferrocene is intriguingly similar and yet different. Similar because e.g. carbon via four electron pair bonds. He did not (in 1916) realise that 8 = 2(1 + 3), and that the next in sequence would be 18 = 2(1 + 3 + 5). That would have to wait for quantum mechanics, and of course inorganic chemists now call it the 18-electron rule (for an example of the 32-electron rule, or 2+6+10+14, as first suggested by Langmuir in 1921[3] (see also here[4]).

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References

1. G. Wilkinson, M. Rosenblum, M.C. Whiting, and R.B. Woodward, "THE STRUCTURE OF IRON BIS-CYCLOPENTADIENYL", Journal of the American Chemical Society, vol. 74, pp. 2125-2126, 1952. http://dx.doi.org/10.1021/ja01128a527
2. G. Wilkinson, "The iron sandwich. A recollection of the first four months", Journal of Organometallic Chemistry, vol. 100, pp. 273-278, 1975. http://dx.doi.org/10.1016/S0022-328X(00)88947-0
3. I. Langmuir, "Types of Valence", Science, vol. 54, pp. 59-67, 1921. http://dx.doi.org/10.1126/science.54.1386.59
4. J. Dognon, C. Clavaguéra, and P. Pyykkö, "Towards a 32‐Electron Principle: Pu@Pb₁₂ and Related Systems", Angewandte Chemie International Edition, vol. 46, pp. 1427-1430, 2007. http://dx.doi.org/10.1002/anie.200604198