The coordination geometry of the metal ion iron, Fe2+ in the active site of hemoglobin is octahedral. Hemoglobin is a protein responsible for transporting oxygen in the blood. It consists of four subunits, each containing a heme group with an iron Fe2+ ion at its center.In the deoxyhemoglobin state without oxygen bound , the iron ion is coordinated to four nitrogen atoms from the porphyrin ring of the heme group, forming a square planar geometry. The fifth coordination site is occupied by a nitrogen atom from a histidine residue called the proximal histidine of the protein. This results in a distorted five-coordinate square pyramidal geometry.When an oxygen molecule binds to the iron ion, the geometry changes to a more regular octahedral coordination. The oxygen molecule binds to the sixth coordination site, opposite the proximal histidine. This binding causes the iron ion to move into the plane of the porphyrin ring, which in turn leads to a conformational change in the protein structure, stabilizing the oxyhemoglobin state.The octahedral coordination geometry allows for the reversible binding and release of oxygen molecules. When hemoglobin encounters an oxygen-rich environment such as in the lungs , the oxygen molecules bind to the iron ions, forming oxyhemoglobin. In oxygen-poor environments such as in tissues , the oxygen molecules are released, and the hemoglobin returns to its deoxyhemoglobin state.This reversible binding and release of oxygen molecules are facilitated by the octahedral coordination geometry, which allows for the necessary structural changes in the heme group and the protein as a whole.