2.4- Pentanedione anion, resonance

The 2,4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=0 bond on the left. The 0=C-C grouping is a typical one for which two resonance structures can be drawn. 

Just as there is a C=0 bond to the left of the lone pair, there is a second C=0 bond to the right. Thus, we can draw a total of three resonance structures for the 2,4-pentanedione anion. 

By learning to recognize such three-atom groupings within larger structures, resonance forms can be systematically generated. Look, for instance, at the anion produced when H+ is removed from 2,4-pentanedione by reaction with a base. How many resonance structures does the resultant anion have ... 

Finally, chelating ligands such as acctylacctonc enjoy resonance stabilization as a result of forming six-membered rings having some aromatic character. Acetylacetone (2.4-pentanedione) coordinates as an anionic enolate ligand ... 

Conclusive proof comes from an investigation of the unsymmetrically deuteriated bis-2,4-pentanedione complex of zinc. The carbonyl resonance of this complex is a single peak and no splittings were observed. This shows that the anion of the zinc complex is a symmetrical single energy minimum and confirms the conclusion that the enol is a rapidly equilibrating mixture of [12a] and [12b]. 

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