Reaction of the 1,2-disilylenes {(DipArAm)Si}2 (DipArAm=[(NDip)2CAr]−, Dip=2,6-diisopropylphenyl, Ar=4-C6H4But (Ar’) 1 a or Ph 1 b) and two abnormal N-heterocyclic silylenes, (DipArAm)SiOCSi{(NDip)2CAr} (Ar=Ar’ 3 a or Ph 3 b) with N2O led to formation of unprecedented examples of uncoordinated silicon analogues of carboxylic acid anhydrides, (DipArAm)(O=)SiOSi(=O)(DipArAm) (Ar=Ar’ 2 a or Ph 2 b). Both compounds have been fully characterized and the mechanism of formation of one explored using DFT calculations. Reduction of sila-acid anhydride 2 a with a dimagnesium(I) compound, [{(MesNacnac)Mg}2] (MesNacnac=[(MesNCMe)2CH]−, Mes=mesityl), led to the one-electron reduction of the anhydride and formation of a magnesium complex of a sila-acid anhydride radical anion [(MesNacnac)Mg{(OSi(DipAr’Am)}2O] 5. A combination of EPR spectroscopic studies and DFT calculations reveal the unpaired electron to largely reside on one of the amidinate ligands of the complex. © 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.