The coordination geometry of the metal ion iron, Fe2+ in the active site of the enzyme hemoglobin is an octahedral geometry. Hemoglobin is a protein made up of four subunits, each containing a heme group with an iron Fe2+ ion at its center. The iron ion is coordinated to four nitrogen atoms from the porphyrin ring of the heme group, forming a planar square. The fifth coordination site is occupied by a nitrogen atom from the imidazole side chain of a histidine residue in the protein proximal histidine . The sixth coordination site is available for binding to oxygen O2 or other ligands.This octahedral coordination geometry plays a crucial role in the function of hemoglobin in oxygen transport. When oxygen binds to the sixth coordination site of the iron ion, it causes a conformational change in the protein structure. This change, known as the "R-state" or relaxed state, increases hemoglobin's affinity for oxygen in the other subunits, allowing it to pick up more oxygen molecules in the oxygen-rich environment of the lungs.When hemoglobin reaches the oxygen-poor environment of the tissues, the bound oxygen molecules are released, and the protein returns to its original conformation, known as the "T-state" or tense state. This change in coordination geometry and protein conformation allows hemoglobin to release oxygen where it is needed and pick it up again in the lungs, effectively transporting oxygen throughout the body.