A selective three-component coupling was employed to fix carbon dioxide to boron guanidinato compounds. The one-pot reaction of carbon dioxide, carbodiimides, and borylamines (ArNH)BC8H14 afforded the corresponding 1,2-adducts {R(H)N}C{N(Ar)}(NR)(CO2)BC8H14. Alternatively, the reaction with p-Me0C6H4NC or 2,6-Me2C6H3NC gave the corresponding isocyanide 1,1-adducts {i-PrHN}C{N(p-Me-C6H4)}(Ni-Pr){CNAr}BC8H14. The molecular structures of products (2,6-i-Pr2C6H3NH)BC8H14 7, {i-Pr(H)N}C{N(p-MeC6H4)}(Ni-Pr)-(CO2)BC8H14 9, {Cy(H)N}C{N(p-MeC6H4)}(Cy)-(CO2)BC8H14 13, and {i-PrHN}C{N(p-MeC6H4)}(Ni-Pr)-{CNR"}BC8H14 (R" = p-MeOC6H4, 2,6-Me2C6H3) 14 and 15 were established by X-ray diffraction. Density functional theory calculations at the M05-2X level of theory revealed that CO2 fixation and formation of the corresponding adduct is exothermic and proceeds via a nonchelate boron guanidinato intermediate.