Cooperative effects between halogen and donor-acceptor interactions are studied in C5H5B center dot center dot center dot NCX center dot center dot center dot NCY complexes, where X = F, CI, Br, I and Y = H, CN, OH, Li. The B center dot center dot center dot N binding distances are found to be longer than those in C5H5B center dot center dot center dot NCX dimers, which means that the formation of the X center dot center dot center dot N interaction weakens the B center dot center dot center dot 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 pi -> pi* orbital interaction between C5H5B and NCX moieties is responsible for the unusual cooperativity of B center dot center dot center dot N and X center dot center dot center dot N interactions. (C) 2017 Elsevier B.V. All rights reserved.