Answer: Fe 2+ (iron II).
- A Fe<sup>2+</sup> (iron II)
- B Mg<sup>2+</sup> according to standard textbooks
- C Cu<sup>2+</sup> in general practice
- D Co3+ as frequently described
Correct answer: A. Fe<sup>2+</sup> (iron II)
Explanation: Haemoglobin's haem group contains Fe<sup>2+</sup> coordinated in a porphyrin ring; it reversibly binds O<sub>2</sub>.
The three common coordination geometries: octahedral (6 ligands), tetrahedral (4 ligands), and square planar (4 ligands in one plane).
Concept context
Study of compounds where a central metal atom is bonded to surrounding ligands. Covers nomenclature, types of isomerism, bonding theories (VBT, CFT), and applications in medicine, photography, and industry.