The mechanisms of reductive functionalization of CO 2 to formamide catalyzed by N-heterocyclic carbene (NHC) were comprehensively studied with DFT calculations. New activation mode with much lower energy barrier than those proposed before was discovered. In this reaction, NHC acts as neither a CO 2 nor a silane activator, but as a precursor of the real catalyst, i.e., the in situ formed ionic liquid [NHCH] + [Carbamate] -. In this loose contact ion pair, the negatively charged O atom of the carbamate anion becomes the new active site, and is free to do nucleophilic attack. When amine is absent, CO 2 will be converted into methanol. In this case, the NHC-CO 2 adduct is the real catalytic species, the active site shifted from the carbene C atom to the negatively charged O atom. These new activation modes follow a pattern of "S N 2@Si-Acceptor", in which the Si-H bond is activated via concerted backside S N 2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO 2 molecule. The advantages of these new activation modes originate from the following points: (1) the ionic liquid [NHCH] + [Carbamate] -and NHC-CO 2 adduct are thermodynamically more stable than NHC; (2) the active site of the NHC catalyst is extended outside a lot. Consequently, the large steric effect between the NHC arms and the substrates in transition state can be avoided to some extent; (3) O atom has good silicon-affinity. In addition, a free CO 2 molecule, whose carbon atom is more electrophilic than those of the CO 2 moieties in NHC-CO 2 adduct and carbamate, acts as an efficient hydride acceptor.