Dispersion Forces, Disproportionation, and Stable High-Valent Late Transition Metal Alkyls

The transition metal tetra- and trinorbornyl bromide complexes, M(nor)(4) (M=Fe, Co, Ni) and Ni(nor)(3)Br (nor=1-bicyclo[2.2.1] hept-1-yl) and their homolytic fragmentations were studied computationally using hybrid density functional theory (DFT) at the B3PW91 and B3PW91-D3 dispersion-corrected levels. Experimental structures were well replicated; the dispersion correction resulted in shortened M- bond lengths for the stable complexes, and it was found that Fe(nor)(4) receives a remarkable 45.9 kcalmol(-1) stabilization from the dispersion effects whereas the tetragonalized Co(nor)(4) shows stabilization of 38.3 kcalmol(-1). Ni(nor)(4) was calculated to be highly tetragonalized with long Ni-C bonds, providing a rationale for its current synthetic inaccessibility. Isodesmic exchange evaluation for Fe(nor)(4) confirmed that dispersion force attraction between norbornyl substituents is fundamental to the stability of these species.