Pnictogen bonding in coordination chemistry

The pnictogen bond (PnB) is a noncovalent attraction between electrophilic pnictogen atoms, originated from the anisotropic distribution of electron density on Pn in a molecular entity, and a nucleophilic (Nu) region in the same (intramolecular) or another (intermolecular) molecular entity: R–Pn⋯Nu (Pn = N, P, As, Sb or Bi; R = electron withdrawing atom or group; Nu = Ha, Ch, Pn, π-system, anion, etc.). Like the halogen and chalcogen bonds, PnB is a directional noncovalent interaction with a preference for a linear geometry, R–Pn⋯Nu angle approaching 180°. In contrast to the halogen and chalcogen atoms, the pnictogen atoms are able to introduce three electrophilic centres on the Pn atom (on account of the existence of three species bonded to the pnictogen atom), which gives an additional advantage in the use of these weak forces in coordination chemistry. In this review we highlight several recent relevant examples, based on X-ray crystal structure analyses, in which PnB is used as a synthon in the construction and design of coordination and organometallic compounds.