The novel complex [RuCl2(PPh3)2(pyrrolidine)] exhibited a square pyramidal (SQP) type geometry, with the amine in the apical position and trans-positioned phosphines. There was activity observed for ROMP of norbornene (NBE) either under air or an inert atmosphere of argon at 25 °C. Saturation profile-fitting describes the curves of yields as a function of time (5-120 min) under either atmosphere. The results were sensitive to the solution volume in the range of 2-8 mL of CHCl3, mainly under air. Quantitative yields under argon for 120 min at any solution volume were obtained. The order of magnitude of 103-105 g mol-1 for Mw and PDI values ranging from 1.6 to 3.5 was measured by SEC. In most of the cases, the ratios of yields from the runs under argon and air were close to the unit, indicating an insignificant loss of reactivity of the active species in the presence of molecular oxygen from air. A parent complex with diethylamine, but with trigonal bipyramidal (TBP) type geometry, was inactive for ROMP. The activity in these types of complexes was compared with that of already reported Ru-amine-based complexes in terms of electronic effects, with an additional discussion involving the synergism in the novel ttt-[RuCl2(PPh3)2(perhydroazepine)(isonicotinamide)] complex.