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Triboracyclopropenyl
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<p>The triboracyclopropenyl fragment is a <a href="/facts/Cyclic_compound/TH2ReKIQ">cyclic</a> structural motif in <a href="/facts/Boron/Pmwgd7Mf">boron</a> chemistry, named for its geometric similarity to <a href="/facts/Cyclopropene/dD0W9xmj">cyclopropene</a>. In contrast to nonplanar <a href="/facts/Boranes/wu3RTzUS">borane clusters</a> that exhibit higher <a href="/facts/Coordination_number/mf48hsoX">coordination numbers</a> at boron (e.g., through <a href="/facts/Three-center_two-electron_bond/1etgQjYh">3-center 2-electron bonds</a> to bridging <a href="/facts/Hydride/v2rlBGMg">hydrides</a> or <a href="/facts/Cation/wrbwhF3z">cations</a>), triboracyclopropenyl-type structures are rings of three boron atoms where substituents at each boron are also <a href="/facts/Coplanarity/dSTa1bcb">coplanar</a> to the ring. Triboracyclopropenyl-containing compounds are extreme cases of inorganic <a href="/facts/Aromaticity/wK0DzRY6">aromaticity</a>. They are the lightest and smallest cyclic structures known to display the bonding and magnetic properties that originate from fully <a href="/facts/Delocalized_electron/jsquhECb">delocalized electrons</a> in orbitals of <a href="/facts/Sigma_bond/CSyEe5CY">σ</a> and <a href="/facts/Pi_bond/3Q6LAGYK">π</a> symmetry. Although three-membered rings of boron are frequently so <a href="/facts/Bent_bond/NQ7odhFQ">highly strained</a> as to be experimentally inaccessible, academic interest in their distinctive aromaticity and possible role as intermediates of <a href="/facts/Borane/pHQMPw5r">borane</a> pyrolysis motivated extensive computational studies by <a href="/facts/Theoretical_chemist/uqcyS5KH">theoretical chemists</a>. Beginning in the late 1980s with <a href="/facts/Mass_spectrometry/RH5WBHJ2">mass spectrometry</a> work by Anderson <i>et al</i>. on all-boron clusters, experimental studies of triboracyclopropenyls were for decades exclusively limited to gas-phase investigations of the simplest rings (ions of B3). However, more recent work has stabilized the triboracyclopropenyl moiety via coordination to donor <a href="/facts/Ligand/6etB3qeW">ligands</a> or <a href="/facts/Transition_metal/0slGWoLQ">transition metals</a>, dramatically expanding the scope of its chemistry. </p>