The acid form of bis(acetylacetone)ethylenedimine, H(2)acacen, was isolated as colorless needle like crystals from condensation reaction of ethylenediamine with acetylacetone at room temperature and fully characterized by MS, IR, H-1- and C-13-NMR spectroscopy, and X-ray structure analysis. The spectroscopic and structural data show the existence of intramolecular N-H...O hydrogen bonding assisted by resonance in the solid protonated form of bis(acetylacetone)ethylenediimine. The intramolecular ionic hydrogen bonding is observed in the enolic form of bis(acetylacetone)ethylenediamine and described by 'resonance assisted hydrogen bonding' model which is essentially a synergetic mutual reinforcement of hydrogen bonding and pi delocalization within the heterodienic system. In this intramolecular ionic hydrogen bonding, the proton is transferred from the enolic oxygen atom to the imine nitrogen atom leaving a negative charge on the oxygen and creating a positive charge on the nitrogen. (C) 2003 Elsevier B.V. All rights reserved.