Posts Tagged ‘18-electron rule’


Sunday, April 17th, 2011
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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 Lewis postulated an octet of electrons as being key to the (quadri)valencies of 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).



  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.
  2. G. Wilkinson, "The iron sandwich. A recollection of the first four months", Journal of Organometallic Chemistry, vol. 100, pp. 273-278, 1975.
  3. I. Langmuir, "TYPES OF VALENCE", Science, vol. 54, pp. 59-67, 1921.