The two-electron reduction of 2,3-dimethylbuta-1,3-diene (DMB) with β-diketiminate and guanidinate substituted dimagnesium(I) compounds has given complexes in which two bidentate amido-magnesium fragments are bridged through the π-system of the DMB dianion, viz. [(LMg)2(μ-DMB)] (L=XylNacnac, [HC(MeCNXyl)2]−, Xyl=2,6-xylyl; or Priso=[(DipN)2CNPri2]−, Dip=2,6-diisopropylphenyl). Similar double reductions of [4]dendralene (4dend) have afforded the complexes, [(LMg)2(μ-4dend)] (L=ArNacnac, Ar=Xyl or mesityl (Mes); or Priso) in which the 4dend dianion is π-coordinated to the bidentate amido-magnesium fragments. Treatment of several such complexes with THF leads to Z- to E-isomerization of the dendralene fragment and formation of purely σ-bonded Mg−C interactions in the THF coordinated products [{(ArNacnac)(THF)Mg}2(μ-4dend)] (Ar=Xyl, Mes or Dip). Reaction of myrcene (Myr) with [{(XylNacnac)Mg}2] proceeds via reductive coupling of Myr to give a previously unknown acyclic, branched C20 tetra-olefin dianion complex [{(XylNacnac)(THF)Mg}2(μ-Myr)2]. Preliminary reactions of [(LMg)2(μ-DMB)] with H2 and/or CO yielded a series of products, including novel magnesium hydride compounds, products derived from couplings of CO with the reduced DMB fragment (viz. magnesium dimethylcyclohexadienediolates) and one magnesium cyclopropanetriolate complex from the magnesium(I) induced coupling of DMB with H2 and CO. © 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.