Spin Crossover during β‑Hydride Elimination in High-Spin Iron(II)– and Cobalt(II)–Alkyl Complexes

It is surprising that rapid β-hydride elimination (βHE) can take place in some high-spin iron(II)– and cobalt(II)–alkyl complexes despite the absence of empty d orbitals. In this study, density functional theory (DFT) is used to analyze the pathways for βHE in alkyl complexes of iron(II) and cobalt(II) supported by β-diketiminate that undergo βHE, and in tris(pyrazolyl)borate (Tp) iron(II)–alkyl complexes that are resistant to βHE. Each reaction pathway includes spin crossover to a transition state with a lower spin and a vacant d orbital; importantly, only the spin crossover accelerated pathway matches experimental rates. The lower spin transition state has a square-planar geometry that is ideal for depopulating one in-plane d orbital that can accept the electrons from the β-hydrogen. The energy of the square-planar transition state is increased by steric bulk around the metal center and by increases in the coordination number at iron, explaining the resistance to βHE in TpFeR. Migratory insertion, the microscopic reverse of βHE, is also accelerated by spin crossover, as shown through an analogous analysis of the insertion of N2H2 into the Fe–H bond of a β-diketiminate supported iron(II)–hydride complex.