A geometry optimization resulted in a triplet spin ground state with the singlet spin much higher in energy at DG = 14.4 kcal mol -1 . This is in agreement with the experimental data reported above of a triplet spin ground state of 7. The optimized geometry (see Figure ) binds HBpin to iron through the hydride with a Fe-H distance of 2.246 Å, while the H-B distance remains short at 1.196 Å. We tested several reaction pathways for nitrile hydroboration, namely starting with C-N bond formation and H-transfer from HBpin. The C-N cleavage pathway was found to be high in energy and did not lead to a stable local minimum. We then investigated hydride transfer from HBpin to the metal or its ligands and attempted to form a bond with either iron or the boron atom of the diphosphinoborane ligand. The lowest energy pathway led to hydride transfer from HBpin to the boron atom of the diphosphinoborane ligand via transition state 3 TS1 with a free energy of activation of DG ‡ = 3.5 kcal mol -1 . The optimized geometry is shown in Figure and displays a large imaginary frequency of i546 cm -1 for the B-H-B stretch vibration, which should have resulted in a large kinetic isotope effect if this barrier was rate-determining. However, the barrier is small and the H-Bpin bond cleavage is not rate-determining in the process. This computational result is in agreement with the experimental observation of an inverse isotope effect for the reaction of HBpin versus DBpin with 7. Structurally, the hydride transfer is relatively central with long B-H distances towards its donor and acceptor atoms of 1.400 and 1.830 Å, respectively. The hydride is relatively close to iron in the transition state (at a distance of 1.587 Å), which shows that the iron atom guides the H-B cleavage step. After the transition state, the system relaxes to structure 3 IM1, which has the hydride bridged between the borane group and iron. Subsequently, the boron atom of the Bpin group attacks the nitrile group via transition state 3 TS2 to form the N-B bond. This step has a significant barrier and a DG ‡ = 17.1 kcal mol -1 with respect to 3 IM1 but leads through a large exothermicity to structure 3 IM2. Consequently, the Bpin transfer to the nitrile will be irreversible. However, the final hydride transfer incurs a significant barrier of DG ‡ = 9.2 kcal mol -1 with respect to the reactant complex for hydride transfer to the carbon atom of the nitrile. As such the highest barriers obtained from the DFT calculations are via 3 TS3 and will be rate-determining, which explains the inverse kinetic isotope effect observed experimentally. The 3 TS2 and 3 TS3 transition states are shown in Figure , and 3 TS2 has a small imaginary frequency (i313 cm -1 ) for the C-N stretch vibration, whereas 3 TS3 displays a somewhat larger imaginary frequency of i768 cm -1 for the final proton transfer.