The multiconfiguration Spin-Coupled approach for the description of the three-center two-electron chemical bond of some carbenium and nonclassical ions

The intrinsically elusive concepts of electronic “delocalization” and “chemical resonance” are briefly reviewed emphasizing their connection with Spin-Coupled (SC) descriptions of electronic structure. Multiconfiguration Spin-Coupled (MC-SC) calculations are performed to describe the three-center two-electron (3c-2e) bonding in some representative carbenium and nonclassical carbonium ions. Within the MC-SC approach, it is found that these cations present significant electronic energy stabilization when described by more than one valence SC spatial orbital configuration. It is shown that it is necessary to have a superposition of two chemical structures to completely span the orbital valence space of these cations. Two characteristic bonding themes are clearly distinguished. One specific to allyl-type carbenium ions and another specific to the nonclassical carbonium ions. In both situations, the 3c-2e bond is described by two chemical structures. The 3c-2e bond present in these carbocations is described clearly within this conceptual framework. The results point out for the robustness of the Spin-Coupled description in yielding a general picture of bonding, even when considering valence-bond type multiconfiguration effects.