Toward the Chemical Structure of Diborane: Electronic Force Density Fields, Effective Electronegativity, and Internuclear Turning Surface Properties

The chemical structure of diborane was elucidated through the superposition of the vector fields of the electron density gradient ∇ρ(<b>r</b>), the electrostatic force <b>F</b>_es(<b>r</b>), and the kinetic force <b>F</b>_<i>k</i>(<b>r</b>), together with the analysis of the cumulative charges of the atoms and pseudoatoms delimited in the aforementioned fields. It was proposed that the <b>F</b>_<i>k</i>-pseudoatomic charge could be employed as a metric for quantifying the ionic component of a related atomic charge. The electron permeability across an internuclear turning surface─specifically, the zero-flux surface in <b>F</b>_<i>k</i>(<b>r</b>)─was characterized by probing it through mapping the total static potential φ_em(<b>r</b>). The conceptualization of post hoc electronegativity was presented for consideration. Our analysis revealed that the ordinary B-H and bent B-μ-H bonds in diborane demonstrate the polar covalent character with minor contributions of the ionic component. The former bond exhibits a greater electron permeability through the internuclear turning surface, indicating a stronger tendency for electron sharing between the corresponding nucleus-dominated regions. The electron density accumulation along the bent B-μ-H bond path diverges from the minimum action trajectories of the forces <b>F</b>_es(<b>r</b>) and <b>F</b>_<i>k</i>(<b>r</b>). This phenomenon can be associated with the structural strain within the angled, three-center two-electron bonding B-μ-H-B. The internuclear B···B paths were identified in <b>F</b>_es(<b>r</b>) and <b>F</b>_<i>k</i>(<b>r</b>), in contrast to ∇ρ(<b>r</b>). This fact, in conjunction with a pronounced electron permeability through the mutual turning surface between the two boron nuclei, implies a certain degree of electron exchange between the boron-dominated pseudoatomic regions. Furthermore, the anomalous [B-]H···μ-H intermolecular polar interactions were described between the strongly negatively charged hydrogen atoms in the monoclinic crystalline β-phase of diborane. In fact, the ionic contributions to the charges of the hydrogen atoms are shown to be relatively small.