Using a bis(N-heterocyclic carbene) ligand system, we have synthesized magnesium complexes bearing redox-active α-diimines and observed structural rearrangements promoted by dynamic N-heterocyclic carbene (NHC) dissociation. The reduction of a bis(NHC)-stabilized magnesium dihalide (iPrNHC)2MgBr2 (1; iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with KC8 in the presence of the respective diimine, affords the doubly reduced compounds (iPrNHC)2Mg(MesDABMe) (2), (iPrNHC)2Mg(MesDABH) (3), and (iPrNHC)2Mg(DippDABMe) (4) (MesDABMe = N,N′-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene, MesDABH = N,N′-
bis(2,4,6-trimethylphenyl)-1,4-diazabutadiene, DippDABMe = N,N′-bis-(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene) as mononuclear five-membered magnesacycles. In contrast to the κ2-diamide coordination in 2−4, (iPrNHC)2Mg(DippDABH) (5; DippDABH = N,N′-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene), prepared under similar conditions, crystallizes as the dinuclear 10-membered magnesacycle [(iPrNHC)Mg(μ DippDABH)]2 (6), where the bridging η1:η1-enediamide ligands are involved in cooperative bonding interactions with the NHC ligands. The diradical complex Mg(DippDABH)2 (7) was also obtained from a solution of 5, which supports an equilibrium between 5 and 6. The rearrangement of 6 to 5 results in an Mg(DAB)2− species that is not stabilized by a Lewis base, which can undergo a disproportionation reaction to form the stable Mg(DAB•−)2 diradical (7). The mechanism for the formation of 6 was evaluated, and a comparative mono(NHC) stabilization of the methylated DAB analogue Mg(DippDABMe) afforded the solid-state coordination polymer [(iPrNHC)Mg-(DippDABMe)·KBr]n (8). The observation of a KBr interaction with the magnesacycle highlights the accessibility to a more Lewis acidic magnesium center upon carbene dissociation from bis(NHC)-stabilized species.