Be(CO) ₃ as a Nontraditional Lewis Base Engaging in Halogen Bonding

The HOMO of Be(CO)₃ resembles a Be lone pair that can donate charge in the context of a halogen bond. This propensity is tested via quantum chemical calculations wherein Be(CO)₃ is paired with XF, XCl, XCN, and XCF₃ (X = I, Br, Cl). In the case of the latter two molecules where X is attached to C, a fully noncovalent halogen bond is formed that varies in strength between 2.2 and 9.7 kcal/mol. The bonding causes a stretch of the internal X-C bond and a red shift of its stretching frequency. Interactions with XF and XCl are much stronger, 40 kcal/mol and higher, containing strong elements of covalency. The bridging X atom in these complexes shifts a good deal toward the Be center in what may be classified as partial transfer or halogen sharing. In contrast to the noncovalent halogen bonds involving XCN and XCF₃, where the interaction is strengthened as X grows larger, the opposite trend is observed for the halogen-shared complexes including XF and XCl.