Isolable Pseudo-monocoordinate Group 14 to 16 Compounds Supported by Hydrindacenyl Ligands

ConspectusMonocoordinate main-group-element compounds have been postulated as key intermediates in diverse chemical transformations and continue to attract significant interest due to their unique bonding and high reactivity. Among them, neutral one-coordinate species stabilized by monoanionic ligands, such as carbynes, nitrenes, and their heavier congeners, exhibit intriguing electronic structures and potential utility as reactive platforms in synthetic chemistry. However, the isolation of such species under ambient conditions remains a formidable task.In this Account, we summarize our recent efforts in the synthesis and characterization of neutral pseudo-monocoordinate group 14-16 compounds, stabilized by sterically encumbered hydrindacenyl ligands. Dehalogenation of hydrindacenyl-tetrelene halides with potassium graphite yielded isolable germylyne, stannylyne, and plumbylyne compounds, representing the first structurally authenticated one-coordinate tetrel radicals. These species possess one unpaired electron populating two nearly degenerate np orbitals and thereby exhibit unquenched orbital angular moment, leading to significant <i>g</i>-factor anisotropy, evidenced by electron paramagnetic resonance (EPR) spectroscopic and wave function based ab initio multireference computational analyses.Photolysis of an azide precursor generated a stable nitrene, and its triplet ground state was conclusively characterized by EPR and superconducting quantum interference device (SQUID) magnetometry to feature a well-defined axial zero-field splitting (ZFS) of <i>D</i> = 0.92 cm^-1 and a nearly vanishing rhombicity <i>E</i>/<i>D</i> ratio. Heavier congeners, including a stibinidene and two bismuthinidenes, were accessed via the reduction of dihalide precursors. Despite possessing triplet ground states, these species are silent in conventional EPR measurements due to extremely large, positive ZFS values (<i>D</i> = 960 cm^-1 for stibinidene; <i>D</i> > 4300 cm^-1 for bismuthinidene), as predicted by ab initio calculations and confirmed by SQUID measurements for stibinidene. Notably, triplet bismuthinidenes are even nonmagnetic at room temperature, due to their gigantic ZFSs that completely depopulate the excited magnetic sublevels. Attempts to synthesize a free phosphinidene led to the isolation of a phosphanorcaradiene containing a strained three-membered PC₂ heterocycle, which could behave as a phosphinidene source in activating a variety of small molecules. Hydrogen abstraction of a tellurol with PbO₂ gave rise to a rare, isolable telluryl radical with an almost orbitally degenerate ground state and field-induced slow magnetic relaxation. This collection of pseudo-one-coordinate species expands the landscape of low-valent main-group chemistry and highlights their potential as functional open-shell platforms for fundamental studies and molecular design.