The results of extensive ab initio configuration interaction (CI) calculations of the spectrum of the [Ni-2(napy)(4)Br-2](+) complex are reported and analyzed. This complex can be seen as the simplest system exhibiting the so-called double-exchange phenomenon. This effect is usually rationalized in the minimal valence space defined by the partially occupied orbitals. The analysis reveals that the leading antiferromagnetic contributions implies the atomic excited (non-Hund) states through a mechanism proposed by Anderson and Blondin but the energy spacings deviate significantly from those predicted by the usually accepted model Hamiltonian. An analytic derivation explains these deviations and an alternative modelization of the spectrum is proposed. The extensive CI calculations also reveal the existence of another mechanism involving low-lying virtual orbitals with a large component on the 4s of the Ni atoms and that strongly stabilizes the upper excited states. (C) 2003 American Institute of Physics.