The cooperative binding of N2 by late transition and main-group metals is a promising strategy for N–N bond weakening and activation. We report the use of activated Rieke magnesium for the reduction of iron and cobalt complexes supported by bulky β-diketiminate ligands. The binding of N2 is accompanied by the assembly of a linear M–NN–Mg–NN–M (M = Co, Fe) core with weakened N–N bonds, as judged by infrared spectroscopy. Both the cobalt and iron complexes require tetrahydrofuran (THF) solvent, because of Mg–THF binding. The cobalt complex can be isolated as a pure solid, but the iron complex is stable only in solution. These results demonstrate the correlation between the binding mode and N–N weakening in heterobimetallic N2 complexes.