The reactions of metallaaziridinium ion pairs [Cp2M(η2-CH2NMePh)][B(C6F5)4] [M=Zr (1), Hf (2)] with 1-hexene and 2-methyl-1-heptene were investigated by means of NMR spectroscopy and DFT calculations. We found that a single insertion of the olefin into the MC bond led to the formation of stable five-membered heterocycles (3–6). The reactions were 1,2-regioselective and afforded two diastereoisomers whose ratio was mainly controlled by the nature of the olefin, whilst the nature of the metal only played a marginal role. In particular, the reaction with 1-hexene was found to be not only more favored than that with 2-methyl-1-heptene from a thermodynamic point of view but it was also more diastereoselective. In all cases, the observed diastereoisomeric ratio was dictated by thermodynamics because the products interconverted into each other, as indicated by EXSY NMR spectroscopy and by DFT calculations.