The addition reactions of nucleophiles to (η4-butadiene)M(CO)3 complexes (M = Fe, Co+) were investigated using a theoretical model recently developed in the framework of the extended Hückel method and allowing evaluation of the intermolecular interaction energy Eint between the organometallic substrate and the hydride ion, chosen as a model reactant. When taking account of both electrostatic and charge-transfer components of Eint, it is seen that this model is able to rationalize the remarkable difference in regioselectivity exhibited by these two complexes towards nucleophilic attack by an identical reactant: the iron complex slightly favours approach to the internal carbon atom of the diene, but the cobalt complex exhibits a marked preference for attack on the external carbon atom. This is in complete agreement with both experimental observations and the results of a previous second order perturbational analysis.