AbstractReactions of a series of magnesium(I) compounds with ethylene, in the presence of an N‐heterocyclic carbene (NHC), have been explored. Treating [{(MesNacnac)Mg}2] (MesNacnac=[HC(MeCNMes)2]−, Mes=mesityl) with an excess of ethylene in the presence of two equivalents of :C{(MeNCMe)2} (TMC) leads to the formal reductive coupling of ethylene, and formation of the 1,2‐dimagnesiobutane complex, [{(MesNacnac)(TMC)Mg}2(μ‐C4H8)]. In contrast, when the reaction is repeated in the presence of three equivalents of TMC, a mixture of the β‐diketiminato magnesium ethyl, [(MesNacnac)(TMC)MgEt], and the NHC coordinated magnesium diamide, [(MesNacnac‐H)Mg(TMC)2], results. Four related products, [(ArNacnac)(TMC)MgEt] (Ar=2,6‐dimethylphenyl (Xyl) or 2,6‐diisopropylphenyl (Dip)) and [(ArNacnac‐H)Mg(TMC)2] (Ar=Xyl or Dip), were similarly synthesised and crystallographically characterized. Computational studies have been employed to investigate the mechanisms of the two observed reaction types, which appear dependent on the substitution pattern of the magnesium(I) compound, and the stoichiometric equivalents of TMC used in the reactions.