We study the bonding characteristics of an imidazole-porphyrin iron (+2) complex [(Im)PorFe] with a second imidazole [Im] by Density Functional Theory. We compared this bond to that formed between (Im)PorFe and other adsorbates, i.e., O-2, NO and CO. The Im nitrogen does not exhibit back-bonding characteristics. This can be attributed to the longer bond length between the imidazole and the heme plane. The interactions with the two histidines are not strong enough to cause significant ruffle-type distortion on the porphyrin plane; on the other hand, the distance of the imidazole groups is limited by the electron repulsion between the hydrogen atoms of imidazole and the porphyrin plane. The resulting energy of the Im-PorFe bond is stronger but is in the same order as that of O-2-PorFe. It is rather manageable for our purpose of designing a smart oxygen storage device. (c) 2005 Elsevier B.V. All rights reserved.