Posts Tagged ‘18-electron rule’


Sunday, April 17th, 2011

The structure of ferrocene was famously analysed by Woodward and Wilkinson in 1952[cite]10.1021/ja01128a527[/cite],[cite]10.1016/S0022-328X(00)88947-0[/cite], 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[cite]10.1126/science.54.1386.59[/cite] (see also here[cite]10.1002/anie.200604198[/cite]).