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 k B(CH 3 ) m (XH 3 ) n (X = Si, Ge, Sn, and n ≥ 2) and BR 5 (R = BH 2 NH 3 , AsH 2 , 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.