Reductions of the in situ generated CO and N2 complexes of Cummins’ molybdenum(III) tris(anilide) complex, viz. [Mo(L){N(Ar’)But}3] (L=CO or N2, Ar’=3,5-dimethylphenyl), using a dimagnesium(I) compound, [{(MesNacnac)Mg}2] (MesNacnac=[HC(MeCNMes)2]−, Mes=mesityl), are reported. Reduction of [Mo(CO){N(Ar’)But}3] afforded a mixture of products, three of which were isolated in low yield and crystallographically characterized. These were a molybdenum(II) tris(carbamoyl) complex, [(MesNacnac)Mg{μ-(O=)[But(Ar’)N]C}3Mo(CO)3] 1, a magnesium(II) carbamate, [{(MesNacnac)Mg[μ-O2CN(Ar’)But]}2] 2 and the 1,3-diamino-1,2,3-trione, But(Ar’)NC3(=O)3N(Ar’)But 3. The mechanisms of formation of these products are speculated upon. The reduction of [Mo(N2){N(Ar’)But}3] with [{(MesNacnac)Mg}2], in the presence of the N-heterocyclic carbene, :C{N(Me)C(Me)}2 TMC, gave the heterobimetallic molybdenum(II) diazenido complex, [(MesNacnac)(TMC)Mg(μ-N2)Mo{N(Ar’)But}3] 4, in a low-moderate isolated yield. Reaction of this with chlorotrimethylsilane afforded an essentially quantitative yield of the known silyldiazenido complex, [(Me3SiN2)Mo{N(Ar’)But}3] 5. © 2021 Wiley-VCH GmbH