A sterically encumbered dinuclear amidinato magnesium hydride 3 with bridging hydrido ligands was synthesized from the metalation of the corresponding amidine 1 with dibutylmagnesium and subsequent treatment with phenylsilane. This complex reacts stoichiometrically with 1 atm of carbon dioxide at room temperature in tetrahydrofuran (THF) to the corresponding bis(thf) adduct of the dinuclear formate complex 4-2 thf with bridging formate anions. This complex is stable in THF at 60 °C for more than 16 hours but decomposes into the amidine 1 upon removal of ligated thf ligands. Complex 3 efficiently catalyzes the reduction of carbon dioxide with 9-BBN, yielding bis(boryl)acetal (R2BO)2CH2, and with silanes H4-nSiPhn (n=1, 2, and 3). The bis(silyl)acetal is quantitatively accessible for Ph3SiH (n=3) whereas during the reduction of CO2 with hydrogen-richer silanes (n=1, 2) siloxanes form and methane evolves.