The metal ion in the active site of hemoglobin is iron Fe , which plays a crucial role in the protein's function as an oxygen carrier. Hemoglobin is a metalloprotein found in red blood cells, and its primary function is to transport oxygen from the lungs to the tissues and organs, as well as to carry carbon dioxide back to the lungs for excretion.The coordination chemistry of the iron ion in hemoglobin contributes to its function as an oxygen carrier in the following ways:1. The iron ion is located in the center of a heme group, which is a flat, ring-shaped organic molecule called a porphyrin. The iron ion is coordinated to four nitrogen atoms from the porphyrin ring in a square planar geometry.2. The fifth coordination site of the iron ion is occupied by a nitrogen atom from an imidazole group of a histidine residue, which is part of the protein chain of hemoglobin. This histidine residue is called the proximal histidine, and it helps to anchor the heme group to the protein.3. The sixth coordination site of the iron ion is initially unoccupied, allowing it to bind to an oxygen molecule O2 . When an oxygen molecule binds to the iron ion, it causes the iron ion to move slightly into the plane of the porphyrin ring. This movement triggers a conformational change in the protein structure, which facilitates the binding of additional oxygen molecules to the other subunits of the hemoglobin molecule. This cooperative binding of oxygen is a key feature of hemoglobin's function as an oxygen carrier.4. The coordination chemistry of the iron ion also plays a role in the release of oxygen from hemoglobin. When hemoglobin reaches the tissues and encounters lower oxygen concentrations, the iron ion changes its coordination geometry, causing the protein to undergo a conformational change that promotes the release of oxygen molecules.5. The iron ion in hemoglobin can also bind to other molecules, such as carbon monoxide CO and carbon dioxide CO2 . The binding of these molecules to the iron ion can affect the protein's ability to carry oxygen, which is why carbon monoxide poisoning can be so dangerous.In summary, the metal ion iron in the active site of hemoglobin plays a critical role in the protein's function as an oxygen carrier. The coordination chemistry of the iron ion allows it to bind and release oxygen molecules, as well as to undergo conformational changes that facilitate cooperative binding and the efficient transport of oxygen throughout the body.