Electronic Structures and Stabilities of Trimethylsilyl-Masked Pnictogen and Chalcogen Complex Anions and Comparisons to Their Hydrides: A Combined Photoelectron Spectroscopy and Computational Study

We report the first electronic structure and stability study on two series of trimethylsilyl (triMS) masked pnictogen (<i>Pn</i>) and chalcogen (<i>Ch</i>) anions <i>Pn</i>(triMS)₂^- and <i>Ch</i>(triMS)^- (<i>Pn</i> = P, As, Sb, Bi; <i>Ch</i> = S, Se, Te) as well as AsH₂^- combining negative ion photoelectron spectroscopy and quantum chemical computations. The <i>Pn</i> and <i>Ch</i> series have exhibited distinct electronic structural patterns due to different numbers of lone pair electrons and triMS ligands. Based on well-defined spectra, precise adiabatic and vertical detachment energies (ADE and VDE) are determined for each of the <i>Pn</i>(triMS)₂^- and <i>Ch</i>(triMS)^- anions, which are all noticeably larger compared to their hydride derivatives, indicating increased electronic stabilities. The magnitude of ADE increments exhibits descending trends from lighter to heavier elements along both <i>Pn</i> and <i>Ch</i> groups, i.e., 0.88, 0.77, 0.53, and 0.44 eV for <i>Pn</i> = P, As, Sb, and Bi and 0.46, 0.31, and 0.15 eV for <i>Ch</i> = S, Se, and Te, respectively. Relativistic multi-reference calculations are carried out to characterize electronic structures of <i>Pn</i>(triMS)₂^-/<i>Ch</i>(triMS)^- anions and corresponding neutral radicals. Only those explicitly including spin-orbit coupling (SOC) effect calculations can reproduce the observed spectral profiles containing heavier <i>Pn</i>/<i>Ch</i> elements, highlighting the important roles of SOC contributions.