A combined experimental and computational investigation was undertaken to investigate the mechanism of aziridination of styrene by the tris(carbene)borate iron(IV) nitride complex, PhB($^t$BuIm)$_3$Fe[triple bond, length as m-dash]N. While mechanistic investigations suggest that aziridination occurs via a reversible, stepwise pathway, it was not possible to confirm the mechanism using only experimental techniques. Density functional theory calculations support a stepwise radical addition mechanism, but suggest that a low-lying triplet ($S$ = 1) state provides the lowest energy path for C–N bond formation (24.6 kcal mol$^{−1}$) and not the singlet ground ($S$ = 0) state. A second spin flip may take place in order to facilitate ring closure and the formation of the quintet ($S$ = 2) aziridino product. A Hammett analysis shows that electron-withdrawing groups increase the rate of reaction σ$_p$ (ρ = 1.2 ± 0.2). This finding is supported by the computational results, which show that the rate-determining step drops from 24.6 kcal mol$^{−1}$ to 18.3 kcal mol$^{−1}$ when (p-NO$_2$C$_6$H$_4$)CH[double bond, length as m-dash]CH$_2$ is used and slightly increases to 25.5 kcal mol−1 using (p-NMe2C6H4)CH[double bond, length as m-dash]CH$_2$ as the substrate.