A new formalism has been developed to evaluate from extended-Huumlckel wave-functions the intermolecular interaction energy Eint between an organometallic substrate S and a model electrophile or nucleophile reactant R. Calculated as the sum of electrostatic, charge-transfer and exchange-repulsion components, Eint is used as a local reactivity index to interpret and predict the regio- and stereoselectivity of electrophilic and nucleophilic addition reactions to organometallics. The methodology developed incorporates molecular graphics as an important ingredient, as the reactivity index is represented using either: (i) a mapping onto the molecular surface by S by means of a color code or (ii) the generation of 3D isoenergy contour surfaces. This model is shown to be able to rationalize the activation of a benzene ring by complexation to the Cr(CO)3 moiety and also to describe adequately the sequential addition of a carbanion nucleophile and of an electrophile to (η6-benzene)tricarbonylchromium derivatives.