Complexing in biological systems

Coordination compounds are the higher-order compounds which are stable in aqueous solutions or dissociate insignificantly.

К4[Fe(CN)6]
Central atom (ion) or complexing agent takes central place in coordination compounds and is usually a positively charged ion
Fe2+
Ligands are ions of opposite charge or neutral molecules that are located (coordinated) around the complexing agent.
CN-

К4[Fe(CN)6]
Inner sphere (coordination entity) is formed by complexing agent and ligands.
[Fe(CN)6]4-
Outer sphere is formed by Ions which are not included in the inner sphere.
K+

Classification of Coordination Compounds
Depending on the electric charge of the inner sphere :
Coordination compounds containing complex cations - [Zn(NH3)4]Cl2
Coordination compounds containing complex anions - K3[Al(OH)6]
Neutral complexes - [Pt(NH3)2Cl2]

Classification of Coordination Compounds
Depending on the nature of ligands:
Acid complexes - [Fe(CN)6]4-
Aqua complexes - [Cr(H2O)6]3+
Hydroxide complexes - [Zn(OH)4]2-
Ammonia complexes - [Cu(NH3)4]2+

Classification of Coordination Compounds
Depending on the chemical compounds class:
Acids - H[AuCl4]
Bases - [Ag(NH3)2]OH
Salts - K2[HgI4]

Classification of Coordination Compounds
Depending on the quantity of central atoms:
Mononuclear - [Cr(NH3)3(H2O)3]Cl3
Polynuclear - [Pt4(OH)4](ClO4)4

When didentate or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it is said to be
a chelate ligand
The coordination compounds in which a ligand is bound with central atom by both donor-acceptor bond and ionic bond are called
chelates