The magnetic coupling between Mn atoms in Mn-2 dimers embedded in a rare gas B matrix is antiferromagnetic but undergoes ferromagnetic transition at a higher temperature or when ionized to the Mn-2(+) state. By use of density functional theory and hybrid functional for exchange-correlation potentials, we show that ferromagnetic transition can also be induced when Mn2 reacts with Cl and/or BO2. Because of their highly electronegative character, both Cl and BO2 draw electrons from the Mn-2 dimer leaving it in a positively charged state. The resulting shrinkage in the Mn-Mn bond brought about by the removal of an antibonding electron causes the magnetic transition. We further show that the coupling between Mn atoms remains ferromagnetic when two Mn2Cl units are allowed to interact with each other. The ability to induce a magnetic transition through a chemical reaction provides a way to synthesize new magnetic materials.