Hemoglobin is a complex of which metal ion?

Hemoglobin is a complex protein that plays a crucial role in the transport of oxygen in the blood. The central metal ion in the hemoglobin molecule is iron, which exists in the +2 oxidation state, denoted as Fe²⁺. This iron ion is integral to hemoglobin's ability to bind oxygen molecules reversibly.

The presence of Fe²⁺ allows for the formation of coordination complexes with oxygen due to its ability to donate electron pairs, facilitating the binding of oxygen to the heme group within hemoglobin. Each hemoglobin molecule can carry up to four oxygen molecules in this manner.

In contrast, the other metal ions listed do not play the same role in hemoglobin. Copper, manganese, and cobalt do not feature in the structure of hemoglobin and do not have the same affinity for oxygen binding as iron does. Thus, the choice of iron as the metal ion in hemoglobin is not only correct but essential for its primary function of oxygen transport in the body.