Relativistic Spin–Orbit Electronegativity and the Chemical Bond Between a Heavy Atom and a Light Atom

Relativistic effects are known to alter the chemical bonds and spectroscopic properties of heavy-element compounds. In this work, we introduce the concept of spin-orbit (SO) electronegativity of a heavy atom, as reflected by an SO-induced change in the interatomic distance between the heavy atom (HA) and a neighboring light atom (LA). We provide a transparent interpretation of these SO effects by using the concept of spin-orbit electron deformation density (SO-EDD). Spin-orbit coupling at the HA induces rearrangement of the electron density for the scalar-relativistically optimized geometry that, in turn, exerts a new force on the LA. The resulting expansion or contraction of the HA-LA bond depends on the nature and electron configuration of the HA. In addition, we quantify the change in atomic electronegativity induced by SO coupling for a series of hydrides, thereby complementing the SO-EDD picture. The trends in the SO-induced electronegativity and the HA-LA bond length across the periodic table of elements are demonstrated and interpreted, and also linked, intuitively, with the SO-induced NMR shielding at the LA.