Diboron Bonds Between BX₃ (X=H, F, CH₃ ) and BYZ₂ (Y=H, F; Z=CO, N₂ , CNH)

The ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations. Due to electron donation from its substituents, the trivalent B atom of BYZ₂ (Z=CO, N₂ , and CNH; Y=H and F) has the ability to in turn donate charge to the B of a BX₃ molecule (X=H, F, and CH₃ ), thus forming a B⋅⋅⋅B diboron bond. These bonds are of two different strengths and character. BH(CO)₂ and BH(CNH)₂ , and their fluorosubstituted analogues BF(CO)₂ and BF(CNH)₂ , engage in a typical noncovalent bond with B(CH₃ )₃ and BF₃ , with interaction energies in the 3-8 kcal/mol range. Certain other combinations result in a much stronger diboron bond, in the 26-44 kcal/mol range, and with a high degree of covalent character. Bonds of this type occur when BH₃ is added to BH(CO)₂ , BH(CNH)₂ , BH(N₂ )₂ , and BF(CO)₂ , or in the complexes of BH(N₂ )₂ with B(CH₃ )₃ and BF₃ . The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components, complemented by smaller polarization energy, while polarization is primarily responsible for the stronger ones.