A combined synthetic and theoretical investigation of N-heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valent  s -block element . The reaction of (NHC)Mg(N(SiMe 3 ) 2 ) 2  ( 1 ) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe 2 BH 3 )(N(SiMe 3 ) 2 ) ( 2 ; NHC-BN = NHC – BH 2 NMe 2 ). In addition to Me 2 N=BH 2  capture at the  NHC C–Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe 2 BH 2 NMe 2 BH 3 ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe 2 BH 3 ) 2 ( 8 ) and (NHC)Mg(NMe 2 BH 2 NMe 2 BH 3 ) 2  ( 9 ) were synthesized, and a dynamic migration of Me 2 N=BH 2  between Mg–N and  NHC C–Mg bonds was observed in  9 . This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the ˉ{NMe 2 BH 2 NMe 2 BH 3 } anion, as well as the capacity of NHCs to reversibly capture Me 2 N=BH 2  in the presence of Lewis acidic magnesium species.