Accounting for the Differences in the Structures and Relative Energies of the Highly Homoatomic np_π−np_π (n ≥ 3)-Bonded S₂I₄²⁺, the Se−I π-Bonded Se₂I₄²⁺, and Their Higher-Energy Isomers by AIM, MO, NBO, and VB Methodologies^‖

The bonding in the highly homoatomic np_π−np_π (n ≥ 3)-bonded S₂I₄²⁺ (three σ + two π bonds), the Se−I π-bonded Se₂I₄²⁺ (four σ + one π bonds), and their higher-energy isomers have been studied using modern DFT and ab initio calculations and theoretical analysis methods:  atoms in molecules (AIM), molecular orbital (MO), natural bond orbital (NBO), and valence bond (VB) analyses, giving their relative energies, theoretical bond orders, and atomic charges. The aim of this work was to seek theory-based answers to four main questions:  (1) Are the previously proposed simple π*−π* bonding models valid for S₂I₄²⁺ and Se₂I₄²⁺? (2) What accounts for the difference in the structures of S₂I₄²⁺ and Se₂I₄²⁺? (3) Why are the classically bonded isolobal P₂I₄ and As₂I₄ structures not adopted? (4) Is the high experimentally observed S−S bond order supported by theoretical bond orders, and how does it relate to high bond orders between other heavier main group elements? The AIM analysis confirmed the high bond orders and established that the weak bonds observed in S₂I₄²⁺ and Se₂I₄²⁺ are real and the bonding in these cations is covalent in nature. The full MO analysis confirmed that S₂I₄²⁺ contains three σ and two π bonds, that the positive charge is essentially equally distributed over all atoms, that the bonding between S₂ and two I₂⁺ units in S₂I₄²⁺ is best described by two mutually perpendicular 4c2e π*−π* bonds, and that in Se₂I₄²⁺, two SeI₂⁺ moieties are joined by a 6c2e π*−π* bond, both in agreement with previously suggested models. The VB treatment provided a complementary approach to MO analysis and provided insight how the formation of the weak bonds affects the other bonds. The NBO analysis and the calculated AIM charges showed that the minimization of the electrostatic repulsion between EI₂⁺ units (E = S, Se) and the delocalization of the positive charge are the main factors that explain why the nonclassical structures are favored for S₂I₄²⁺ and Se₂I₄²⁺. The difference in the structures of S₂I₄²⁺ and Se₂I₄²⁺ is related to the high strength of the S−S π bond compared to the weak S−I σ bond and the additional stabilization from increased delocalization of positive charge in the structure of S₂I₄²⁺ compared to the structure of Se₂I₄²⁺. The investigation of the E₂X₄²⁺ series (E = S, Se, Te; X = Cl, Br, I) revealed that only S₂I₄²⁺ adopts the highly np_π−np_π (n ≥ 3)-bonded structure, while all other dications favor the π-bonded Se₂I₄²⁺ structure. Theoretical bond order calculations for S₂I₄²⁺ confirm the previously presented experimentally based bond orders for S−S (2.1−2.3) and I−I (1.3−1.5) bonds. The S−S bond is determined to have the highest reported S−S bond order in an isolated compound and has a bond order that is either similar to or slightly less than the Si−Si bond order in the proposed triply bonded [(Me₃Si)₂CH]₂(ⁱPr)SiSi⋮SiSi(ⁱPr)[CH(SiMe₃)₂]₂ depending on the definition of bond orders used.