The coordination chemistry of iron in hemoglobin plays a crucial role in the transportation of oxygen in the blood. Hemoglobin is a protein found in red blood cells, and it contains four subunits, each with an iron-containing heme group. The iron atom in the heme group is responsible for binding to oxygen molecules and transporting them throughout the body.The iron atom in the heme group is in the +2 oxidation state Fe2+ and has a coordination number of six. Four of these coordination sites are occupied by nitrogen atoms from the porphyrin ring, which is an organic molecule that forms the heme group. The fifth coordination site is occupied by a nitrogen atom from a histidine residue, which is part of the protein chain of hemoglobin. This histidine residue helps to stabilize the iron atom and plays a crucial role in the binding and release of oxygen.The sixth coordination site is where oxygen binds to the iron atom. When an oxygen molecule O2 approaches the heme group, it forms a coordinate covalent bond with the iron atom, occupying the sixth coordination site. This binding causes a slight change in the geometry of the iron atom, which in turn leads to conformational changes in the hemoglobin protein. These changes enable hemoglobin to bind more oxygen molecules, a phenomenon known as cooperativity.As blood circulates through the body and reaches oxygen-depleted tissues, the concentration of oxygen decreases. This decrease in oxygen concentration causes the iron atom to release the bound oxygen molecule, which can then be used by the cells for various metabolic processes. The release of oxygen also triggers conformational changes in the hemoglobin protein, making it less likely to bind to additional oxygen molecules. This ensures that oxygen is released where it is needed most.In summary, the coordination chemistry of iron in hemoglobin allows for the reversible binding and release of oxygen molecules, enabling efficient transportation of oxygen throughout the body. The iron atom's ability to change its coordination geometry in response to oxygen binding and release is crucial for this process.