In a theoretical investigation of the structure and reactivity of indole derivatives of tricarbonylchromium(0), we have used two different semi-empirical quantum chemical models derived from the extended Hückel molecular-orbital (EHMO) formalism. The first one, based on the atom-superposition and electron-delocalization (ASED) method, is used to optimize the geometry of the systems; it is shown to lead to results in satisfactory agreement with experiment in the case of the complex for which X-ray structural data are available, the average errors being 0.03-0.05 Å for bond distances and 5° for bond angles. The second one consists of a local reactivity index, made of the intermolecular interaction energy between the organometallic substrate and a model reactant. It is seen that this procedure is able to reproduce the experimental trends as to the most reactive regions of the systems investigated towards nucleophilic and electrophilic addition reactions.