| Abstract: | The vibrational spectra of molecules labeled with 2H, 13C, and 18O show that delocalization of double and single bonds in the six-membered ring is complete in all the metal chelates investigated of 2,4-pentanedione (acetylacetone), but not in 2,4-pentanedione itself. Mercury, on the other hand, is bonded to the central C atom of the ligands. The NMR spectra of the metal chelates of 3-mesityl-2,4-pentanedione and 3-anthryl-2,4-pentanedione show that the chelate rings have no magnetic anisotropy comparable with that of benzene. A critical appraisal is made of the questions whether it is at all justifiable to attribute any “aromatic character” to such molecules and why all comparisons with the chemical reactivity of benzene have so far led to controversial conclusions concerning the character of the bonding in the metal chelates of 2,4-pentanedione. |
