To probe the mechanism further, kinetic isotope effect (KIE) experiments were conducted and returned a KIE value of 0.3 using DBpin in replacement of HBpin. The observation of the inverse KIE (i.e. KIE = kH/kD < 1) for the hydroboration of benzonitrile using complex A implies the existence of an intermediate in a multistep reaction prior to the rate-determining step (RDS). Specifically, the observed KIE suggests that reductive elimination could be the RDS for this process, see Scheme 4. The reductive elimination (kre) in this transformation can be considered as a composite of the effect of substituting deuterium for hydrogen on the individual rate constants for the reductive coupling (krc) to form the borylimine complex (9), the oxidative cleavage (koc) to regenerate the borohydride-ferratoimine complex (8), and the dissociation (kd) of the boryl-imine. If the dissociation of the boryl-imine from the iron center is facile, the isotope effect on R.E will therefore be dominated by the ratio of the reductive coupling step to the oxidative addition and this ratio can be considered as being identical to the equilibrium isotope (EIE) for the interconversion of 8 and 9. Based on the inverse EIE observed, this is suggestive that the deuterium prefers to be located on the iminyl carbon over the iron and that the oxidative addition of the C-D bond back to 8 would be much slower compared to the hydrogen isotopologue (Scheme 4). A reaction profile study was conducted using standard conditions for the hydroboration of benzonitrile (0.33 M) with HBpin (0.67 M) by A (0.003M) at differing temperatures (313, 323, 333, and 343 K). The activation parameters were determined from the Arrhenius and Eyring analysis plots, with DS ‡ , DH ‡ , and Ea values of 8.36 (±0.64) cal K -1 mol -1 , 27.57 (±0.2) kcal mol -1 and 28.22 (±0.2) kcal mol -1 , respectively. Little can be deduced by the microscopic activation energy and enthalpic considerations, however the near zero value for entropic contribution suggests components of the reaction are pre-assembled prior to the rate-limiting step.