Theoretical Design of Stable Pentacoordinate Boron Compounds

Through theoretical computations, we found that boron can form thermodynamically stable pentacoordinate compounds. Pentacoordinate boron (penta-B) is just hypercoordinate but not hypervalent because it forms only four covalent bonds, of which at least one is a multicenter bond. Being electron deficient, to be pentacoordinate, at least two of its bonding atoms should have low electronegativity. Penta-B can be formed in H_kB­(CH₃)_m(XH₃)_n (X = Si, Ge, Sn, and n ≥ 2) and BR₅ (R = BH₂NH₃, AsH₂, and BeH). Based on a systematic investigation of these model compounds, we designed three thermodynamically stable penta-B compounds that can potentially be synthesized by hydrogenating their tricoordinate counterparts under mild reaction conditions.