Ab initio calculations have been performed in order to study the binding of a singly charged iron cation with methane, ethane, and propane. All three adducts are predicted to possess a quartet ground state with the sextet equilibrium structures lying just slightly higher in energy. The calculated binding energies are found to increase with increasing size of the alkane and amount to 13.1, 16.7, and 21.9 kcal/mol for the methane, ethane, and propane adduct, respectively. These values confirm the experimental data available for these molecular complexes. For all three adducts the quartet and sextet potential energy surfaces are found to intersect at low energies with respect to the quartet ground state and the sextet equilibrium structures. (C) 1997 American Institute of Physics.