DFT calculations have been carried out on the cationic species for the two different Brookhart's catalyst systems: [{ArN=CH-HC=NAr}NiR "](+) (3a) and [{ArN=CMe-MeC=NAr}NiR "] + (3b) (where Ar = {2,6-C6H3(Me)(2)} and R " = Me). These calculations reveal that the conformation of aryl groups attached to nitrogen atoms could provide a suitable explanation for the large experimental differences found in the ethylene polymerization activity.