Zeise, Liebig, Jensen, Hückel, Dewar, and the Olefin π‐Complex Bonds

Zeise's salt, KPt(C2H4)Cl3, was the first characterized organometallic compound; it was also the first olefin π‐complex. It was published in 1825–1830 in the middle of a fight between Dumas on the one hand and Berzelius and Liebig on the other, who defended the etherin (ethylene) and radical theories, respectively. Although Zeise's formulation as a compound containing ethylene was vindicated, the fight went on for many years. This was a time when the theories of organic chemistry were being developed, before any clear understanding of the nature of molecules, bonding, and structure. Zeise thought of the structure of his salt as a product of the addition of PtCl2 to ethylene. Jensen assumed a central bonding to ethylene but needed theoretical assistance to explain it. His attempt to obtain such an explanation from Hückel failed, and it was Dewar who explained the nature of π‐complexes in molecular orbital terms in 1951.