A theoretical analysis of complex formation of benzene with alkaline and alkaline-earth cations is presented. Two types of molecular structures are found from the topology of the electron density, rho(r). Charge redistribution upon complex formation is accounted for by the properties of rho(r) and the quantum topological atoms, revealing the differences between alkaline and alkaline-earth complexes. The long-range energy, calculated with multipoles of the atoms in a complex, allows to conclude that polarization plus charge transfer are essential to recover the ab initio interaction energies for alkaline cations, whereas for the other complexes additional effects such as covalent bonding are also relevant. (c) 2006 Elsevier B.V. All rights reserved.