To test the effect of the metal catalyst on the reaction mechanism, we also explored the reaction of HBpin with benzonitrile in the absence of the iron complex. The subsequently calculated proton transfer from HBpin to the nitrogen atom of benzonitrile has a barrier of DG ‡ = 61.4 kcal mol -1 with respect to isolated reactants. This barrier is high in free energy and will not be able to proceed at 323 K, hence the metal complex provides a low-energy transition state for the reaction between HBpin and benzonitrile. Next, we took the 3 IM3 optimized geometry and manually added another HBpin molecule, and optimized the structure as 3 IM3', see Figure . The orientation of the second HBpin in the complex 3 IM3' is similar to that in 3 RE, with the hydride pointing towards iron. The potential energy profile for the second hydroboration step from 3 IM3' is shown in Figure . The first hydride transfer via 3 TS4 has a free energy of activation of DG ‡ = 21.2 kcal mol -1 , which is significantly higher in free energy than that reported above for 3 TS1. This likely stems from the stereochemical interactions around the iron center that make the borane less accessible than in cycle 1 above. The transition state is highly central with long B-H and H-Bpin distances of 1.403 and 1.736 Å, respectively. The imaginary frequency for hydride transfer is small, i.e. i634 cm -1 , which is of similar value as the one seen for 3 TS1.