The hetero [2+2+2] cycloaddition between two alkynes and one nitrile could proceed either by a) a homocoupling mechanism in which the two alkynes link together first, or b) a heterocoupling mechanism in which one of the alkynes and the nitrile link together first. Correlation with a previous report suggests that the heterocoupling path is indeed operative. To shed some light to the interactions of the chiral Ni(0)NHC complex with the substrates, we synthesized Ni-nitrile complex Ni4 via complexation of L10 with Ni(COD)2 and p-methoxybenzonitrile in 90 % yield (Fig. ). Evidenced by crystal structure analysis, Ni4 exhibits a dimeric structure with planar geometry. Each nitrile binds to two nickel atoms in η 1 and η 2 modes. The R-C-N bond angle is changed from 180° of the former nitrile to 134.12° in the complex indicating substantial π-back-bonding, in line with a strong d 8 Ni(II) metallaazirene character. (50) Ni4 is a competent catalyst and when used instead of Ni3, an identical reaction yield and enantiomeric ratio was observed (Fig. ), suggesting that the styrene ligands of Ni3 likely have a pure spectator role in the later catalytic cycles. No reaction occurred between precatalyst Ni3 and p-methoxybenzonitrile (Fig. ), indicating that a) nickel has a stronger affinity to styrene than nitrile. b) coordination of the nitrile is contingent upon ligand exchange of styrene with alkyne substrate. Exposing Ni4 to styrene did not result in formation of Ni3, but lead to complex decomposition instead (Fig. ). This behavior is further indicative of the Ni(II) character of Ni4 species and suggests that the formation of the metallaazirene species is irreversible. In the absence of a nitrile, Ni3 cleanly catalyzes alkyne trimerization. However, in the presence of nitrile, the formation of pyridine is largely favored. This indicates that pyridine formation proceeds much faster than cyclotrimerization of alkyne (Fig. ). With the heterocoupling mechanism operative, the enantiodetermining step is the selective migratory insertion of one of the enantiotopic alkynes into the Ni(II) metallaazirene. The crystal structure of Ni4 shows that the nitrile substrates are perfectly sandwiched in the middle of chiral side arms with a planar geometry (Fig. ), suggesting that the chiral side arms of the carbene can effectively discern stereochemical information from the dialkyne substrate to achieve enantioselectivity. In sum, these observations enable us to propose a mechanism for the enantioselective hetero [2+2+2] cycloaddition (Fig. ). To initiate a first cycle, ligand exchange occurs on Ni3, whereas a dialkyne substrate and nitrile replace both styrenes yielding intermediate I. Subsequently, oxidative cyclization occurs, leading to the formation of a metallaazirene intermediate. An enantiodetermining migratory insertion of the coordinate alkyne towards the metallaazirene species proceeds. The orientation of group R 1 of the dialkyne substrate is paramount for the selectivity. The substituent R 3 on the nitrile has significantly less impact on the selectivity due to its distance from the chiral sidearm. The substrate catalyst complex adopts an orientation in which the substituent R 1 is favorably placed to the least hindered position leading preferentially to intermediate II 1