Why Is the Nickel(II) Diphenyldiimine Complex the Best Catalyst for Polymerization of Ethylene in Three Kinds of Cationic Nickel(II) Complexes, [Ni(CH3)L]+ (L = Diphenyldiimine, 2,2‘-Bipyridine, or 1,2-Diphosphinoethane)? A Theoretical Study

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Ligand effects in the insertion of ethylene into the Ni(II)−CH3 bond of [Ni(CH3)L]+ {L = diphenyldiimine (PhNCHCHNPh; dpdiim), diimine (HNCHCHNH; diim), 2,2‘-bipyridine (bpy), 1,2-bis(dimethylphosphino)ethane (Me2PCH2CH2PMe2; dmdipe), or 1,2-diphosphinoethane (H2PCH2CH2PH2; dipe)} were theoretically investigated with the DFT method. Ethylene is easily inserted into the Ni(II)−CH3 bond with a moderate activation barrier with L = diim, dpdiim, dipe, and dmdipe but with a larger activation barrier with L = bpy. This is reasonably interpreted in terms that the coordinate bond of bpy is less flexible than those of the others because of its well-conjugated electron system. Next, coordination of ethylene with the nickel(II) propyl complex [Ni(CH2CH2CH3)L]+ involving β-H agostic interaction takes place with a considerably large exothermicity with L = dpdiim, diim, bpy, and dipe but with much less exothermicity with L = dmdipe. Since the propagation process needs the next coordination of ethylene, the nickel(II) d...