The coordination geometry of the metal center in the heme group of myoglobin is octahedral. The metal center is an iron II ion Fe2+ that is coordinated to four nitrogen atoms from the porphyrin ring in a planar arrangement, forming the base of the octahedron. The fifth coordination site is occupied by a nitrogen atom from a histidine residue in the myoglobin protein, known as the proximal histidine. The sixth coordination site, which is opposite to the proximal histidine, is the one that binds and releases oxygen.This octahedral geometry facilitates the binding and release of oxygen by allowing the Fe2+ ion to coordinate with the oxygen molecule at the sixth coordination site. When oxygen binds to the iron II ion, it forms a bent Fe-O-O angle, which weakens the Fe-O bond and makes it easier for oxygen to be released when needed. Additionally, the binding of oxygen causes a small change in the geometry of the iron II ion, pulling it slightly into the plane of the porphyrin ring. This change in geometry is transmitted through the proximal histidine to the myoglobin protein, which can then respond to the presence or absence of bound oxygen.In summary, the octahedral coordination geometry of the metal center in the heme group of myoglobin allows for the reversible binding and release of oxygen, which is crucial for the protein's function in transporting and storing oxygen in muscle tissue.