Ligand

It is a molecule or ion that is coordinated to a central atom in a complex compound by a coordination bond, also known as a ligand. Chemical bonds in complex compounds form due to the donor-acceptor interaction between the ligands and the complexing agent. If the central atom is a positive metal ion, it is usually an electron pair acceptor because it has empty orbitals. The ligands fill the empty orbitals of the complexing agent with electron pairs, acting as electron donors. Ligands are negative ions: CN^–, Cl^–, I^–, SCN^–, OH^–, etc., or polyatomic (H₂O, NH₃, CO, NO) and non-polyatomic (I₂, C₆H₆) molecules. In the formation of the complex ion [Fe(CN)₆]^4–, the complexing agent and electron acceptor are Fe²⁺, while the ligands CN^– form chemical bonds with it using their electron pairs. Under the influence of ligands, the 3d level electrons of the iron (II) ion pair up, creating two vacant orbitals at this level occupied by the electron pairs of the cyanide ions. Each CN^– ion contributes one electron pair to the bond. In total, six donor-acceptor bonds are formed.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė