The crystal structures of (eta(5)-C5H5)W(CO)(3)(eta(1)-N-maleimidato) and (eta(5)-C5H5)Fe(CO)(2)(eta(1)-N-maleimidato) complexes were determined by single crystal X-ray diffraction. The molecular geometries of both structures are compared with those of the Mo analog of the W complex and ethyl-N-maleimide in order to find a relation between the geometrical features and the rate constants of the addition reaction of the sulfhydryl group of biomolecules to the ethylenic bond of the maleimidato fragment. For a deeper insight into the problem DFT calculation were performed. An analysis of atomic charges, using the CHELPG scheme, and of theoretical electron density function, using the AIM theory, was performed. In the (eta(5)-C5H5)W(CO)(3)(eta(1)-N-maleimidato), likewise in its Mo analog, the carbonyl (...) carbonyl interaction was found both for experimental and calculated structures. It is probably the first approach to explain this type of intramolecular interactions acting in organometallic compounds. This interaction can play the essential role in the reaction mechanism of nucleophilic addition to the maleimidato moiety. The AIM investigations indicate also the differences in the character of bonding between the eta-N-maleimidato ligand and the central metal atom. (c) 2006 Elsevier B.V. All rights reserved.