Cooperative effects between halogen and donor-acceptor interactions are studied in C5H5B•••NCX•••NCY complexes, where X=F, Cl, Br, I and Y=H, CN, OH, Li. The B•••N binding distances are found to be longer than those in C5H5B•••NCX dimers, which means that the formation of the X•••N interaction weakens the B•••N bonds. This abnormal cooperativity effect is further investigated by means of molecular electrostatic potential, electron density and natural bond orbital analyses. It is found that the back-bonding π→π* orbital interaction between C5H5B and NCX moieties is responsible for the unusual cooperativity of B•••N and X•••N interactions.
