A previously proposed theory for electron exchange between a metal electrode and a diatomic molecule is generalized to the case where the two atoms are not equivalent. The Green's function for the molecule is derived and used to calculate the densities of states of the atoms and of the molecule. A few examples demonstrate the effect of the interaction with a wide metal sp-band and with a narrow d-band. It is suggested that the formalism can be linked to density-functional theory in order to perform quantitative calculations for particular reactions. (C) 2011 Elsevier B.V. All rights reserved.