  We investigate the role of exchange bridges in molecular magnets. We explore
their effects on the distribution of the valence electrons and their
contribution to the exchange processes. The present study is focused on a
spin-half dimer with nonequivalent exchange bridges. Here, we derive an
effective Hamiltonian that allows for an accurate estimation of the observables
associated to the magnetic properties of the magnet. Our results are compared
to those obtained by means of the conventional Heisenberg model that usually
fails.
