Kinetic and Potential Energy Contributions to a Chemical Bond from the Charge and Energy Decomposition Scheme of Extended Transition State Natural Orbitals for Chemical Valence.

This work provides novel physical insight into the nature of a chemical bond by exploring qualitative and quantitative relations between the natural orbitals for chemical valence (NOCV)-based deformation density bonding channels Δρ i ( i = σ, π, δ, etc.) and the corresponding kinetic Δ Ti and potential energy Δ Vi contributions within the charge and energy decomposition scheme ETS-NOCV implemented in the Kohn-Sham-based Amsterdam Density Functional (ADF) package. It is determined that interfragment dative and covalent-type electron charge reorganizations upon formation of a series of strong and weak bonds employing main-group elements are due to lowering of the negative kinetic energy contributions, as opposed to the intrafragment polarizations (e.g., hyperconjugations in ethane), which are, in contrary, driven by the potential energy (electrostatic) component. Complementary, formation of π-contributions in N2 is accompanied by lowering of both kinetic and potential energy constituents. Remarkably, well-known globally stabilizing back-donation (M → ligand, where M is a transition metal) and donation (ligand → M) processes, ubiquitous in organometallic species, have been discovered for the first time to be driven by the opposite Δ Ti/Δ Vi mechanisms, namely, the former contribution is associated with the negative kinetic term (which outweighs the positive potential energy), whereas the latter charge delocalization into electrophilic transition metals leads to an attractive electrostatic stabilization (and positive kinetic energy).