Toward the Chemical\nStructure of Diborane: Electronic\nForce Density Fields, Effective Electronegativity, and Internuclear\nTurning Surface Properties

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