Coordination chemistry is a branch of chemistry that deals with compounds formed by formation of coordinate covalent bond between a metal and a neutral or negatively charged molecule called ligand. Although this is a part of inorganic chemistry but coordination chemistry plays very crucial role in biological systems. Biological system is a diverse system. Varieties of metal ions are found in the biological systems and so it is expected that coordination compounds will be found in the system. Roles of few such coordination compounds are listed below.

1. Metalloproteins: Metals are commonly found as natural constituent of proteins. Nature has learned to use the special properties of metal ions to perform a wide variety of specific functions associated with life processes. Examples of such metalloproteins and their roles in biological processes are as follows.

1.1 Hemoglobin and myoglobin: Myoglobin is monomeric complex of Iron-porphyrin whereas hemoglobin is tetrameric complex of same unit. Myoglobin acts as oxygen storage protein whereas hemoglobin can store and transport proteins. This protein contains Iron atom that has coordination number six that is like a man with six hands. In the metalloprotein only five atoms coordinate and the sixth position remains vacant that means five hands are blocked but the sixth hand is empty and so this hand can hold a molecule. That is a ligand from outside can coordinate at the sixth coordination site. Oxygen molecule can reversibly bind at the sixth position. Since the binding with oxygen is reversible that is two way process so it can release the oxygen molecule whenever necessary. Thus hemoglobin transports oxygen in blood. It takes oxygen from air in the lungs and deliver it to myoglobin which are found in tissues where myoglobin stores the oxygen and releases during respiration.

1.2 Electron transfer proteins: This kind of metalloproteins act as electron carriers during redox reactions that is reactions occurring with transfer of electrons in biological system. Such biological current carriers usually pass electrons to or from enzymes that require redox chemistry in order to perform a specific function. Two such metalloproteins widely encountered in biological systems are cytochromes and iron-sulfur clusters. In iron-sulfur clusters the iron atom is coordinated by cysteine sulfur as well as inorganic sulfide sulfur whereas cytochrome is a complex of iron-porphyrin system that has resemblance with the structure of hemoglobin used for oxygen transport. Both of these metalloproteins are situated in the mammalian respiratory chain and plays crucial role in the transportation of electrons from one substance to another in the respiratory chain during respiration process.

2. Metalloenzymes: Metalloenzymes are a subclass of metalloproteins that perform specific catalytic functions. The metal centre presents in such kind of molecules, catalyze the required chemical transformation. Different kinds of metalloenzymes are found in the biological system. They are classified according to their functions as follows:

2.1 Hydrolytic enzymes: These type of proteins catalyze addition or removal of water in a substrate molecule. Notable example includes carbonic anhydrase which promote hydrolysis of CO2; peptidases and esterases which catalytically hydrolyze carbonyl compounds; and phosphatases which catalytically cleavage the phosphate esters. In most of the cases the metalloenzyme contains Zn²⁺ ion. Presence of Zn²⁺ ion helps in lowering the pK_a of coordinated oxygen containing ligands, such as the above said substrates, and also can avoid the potential for undesired electron transfer chemistry, since divalent zinc does not have any readily accessible redox states. Other metal ions found in hydrolytic enzymes,  also known as hydrolases are Mn²⁺, Ni²⁺, Ca²⁺, and Mg²⁺. All these metal ions have the ability to avoid redox activity and only alters the pKa value of the oxygen containing ligands so that addition or removal of water molecule in a substrate can be catalyzed.

2.2 Two electron redox enzymes: Many metalloenzyme catalyze reactions that involve either oxidation or reduction of substrate. These reactions are generally two electron redox process. Most common reactions of this type is addition of oxygen atom to the substrate. For example oxidation of hydrocarbons to alcohols catalyzed by cytochrome P-450 enzymes containing iron-porphyrin moiety as the active centre, ortho-hydroxylation of phenolic substrates catalyzed by tyrosinase, which contains a dsinuclear copper in the active site etc. Addition of oxygen is two electrons oxidation process and in biological system these processes are catalyzed by such metalloenzymes due to the presence of redox active metal centres in the active site of the enzyme. Some metalloenzymes remove oxygen atoms from a substrate. This process is accompanied by two electrons reduction and are catalyzed by metalloenzymes, For example nitrate reductase which catalyzes the reduction of nitrate to nitrite and the redox active metal in the enzyme that catalyzes the process is molybdenum.

2.3 Multielectron pair redox enzymes: Metalloenzymes also take part in multi electron pair redox transformations. One of most important such reaction occurring in the respiratory chain is the conversion of oxygen molecule into water that involve four electrons transfer process. Cytochrome c oxidase, a highly complex enzyme containing two copper and two iron-porphyrin centres, catalyzes the reduction of oxygen molecule into water. Another iron containing enzyme catechol dioxygenase cleaves the aromatic ring of catechol and this is also a multi electron transfer process.

3. Metal complexes as medicines: Many metal complexes can also be used as drugs in the treatment of chronic diseases. Cisplatin is a complex of platinum that is used as anticancer drug; Auranofin is an oral rheumatoid arthiritis drug which is a complex of gold: Cardiolyte is a technicium containing complex used as heart imaging agent. Calcium-edta (edta = ethylenediaminetetraacetic acid) complex is injected in our body to remove lead poisoning. Moreover metal ions like Hg²⁺ have been commonly used for the treatment of syphilis, Mg²⁺ for intestinal disorders, and Fe²⁺ for anaemia, over the centuries.

In conclusion we can say that coordination chemistry and coordination complexes or metal complexes play a very crucial role in different biological processes and biochemical field. In this way metal complexes take a part in sustaining the life in the earth.