Dispersion Effects in Stabilizing Organometallic Compounds: Tetra-1-norbornyl Derivatives of the First-Row Transition...

In 1972 Bower and Tennett first synthesized a series of tetra-1-norbornyl derivatives, (nor)₄M, of the first-row transition metals from titanium to cobalt. These were found to be exceptionally stable for homoleptic metal alkyls containing only metal-carbon σ-bonds. The theoretical energies for the dissociation of 1-norbornyl ligands from these unusually high oxidation state organometallics through the reactions (nor)₄M → (nor)₃M + nor^• and (nor)₄M → (nor)₂M + nor-nor indicate that dispersion effects play an important role in determining their exceptional stability. Thus, all of the (nor)₄M (M = Ti to Cu) derivatives are viable with respect to 1-norbornyl radical dissociation when the London dispersion effect is considered. However, (nor)₄Cu becomes disfavored if the dispersion correction is ignored. Thus, the stability of the (nor)₄M molecules is seen to arise from the favorable combination of steric and dispersion force effects of the four 1-norbornyl groups tetrahedrally disposed around the metal atom and maximizing the dispersion attraction between them in a spherical hydrocarbon structure with a central metal atom. The tri-1-norbornyl derivatives (nor)₃M appear be disfavored with respect to disproportionation into (nor)₄M + (nor)₂M. This is consistent with the experimental syntheses of the (nor)₄M (M = Cr to Co) derivatives with the metal in the +4 oxidation by reactions with 1-norbornyllithium with metal halides in the +2 or +3 metal oxidation states. Both the OPBE method and the BPW91 method predict high-spin states for the d² and d³ complexes (nor)₄Cr and (nor)₄Mn but low-spin states for (nor)₄Fe and (nor)₄Co, consistent with experiment.