Calculated as the sum of electrostatic, charge-transfer and exchange-repulsion components derived from extended Hückel wavefunctions, the intermolecular interaction energies between an organometallic substrate S and an incoming reactant R are used as a reactivity index to describe electrophilic and nucleophilic addition or substitution mechanisms. In order to facilitate the interpretation, the values of this local index are mapped onto the molecular surface of S by means of a standard colour or grey-scale code, which allows the possible directions and sites of attack to be visualized rapidly. When applied to important reaction mechanisms such as (i) protonation of ferrocene and iron pentacarbonyl and (ii) nucleophilic attack on arene-V (CO)4+ and butadiene Fe(CO)3, this combination of quantum-chemical and molecular-graphics techniques is shown to describe adequately their high regioselectivity.