Theoretical ab initio studies of neutral, cationic and anionic Cr-2, Mn-2, and CrMn dimers have been carried out to explore the progression of magnetic coupling with the number of electrons. It is shown that while Cr-2 and Cr-2(-) have antiferromagnetically coupled atomic spins, Cr-2(+) has a ferromagnetic ground state closely followed by an antiferromagnetic state. On the other hand, all Mn-2 dimers are ferromagnetic, irrespective of the charge. The neutral CrMn is ferrimagnetic while the charged CrMn are antiferromagnetic. In all cases, the charged dimers are found to be more stable than the neutral ones. The results are compared with available calculations and experiments and the difficulties associated with theoretical description and the experimental interpretations are discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)30311-7].