Which of the following is incorrect?

Chlorophyll, the green pigment in plants, is the complex of cobalt

The correct answer is option (2) Chlorophyll, the green pigment in plants, is the complex of cobalt.

Let us delve into each statement and clarify why the incorrect one is incorrect.

1. Haemoglobin, the red pigment in blood, is a complex of iron and acts as an oxygen carrier

The statement is correct. Hemoglobin is a protein complex that contains iron. The iron is part of the heme group, which is crucial for oxygen binding. Each hemoglobin molecule can bind up to four molecules of oxygen. The iron in the heme group is what allows hemoglobin to pick up oxygen in the lungs and release it in the tissues.

2. Chlorophyll, the green pigment in plants, is the complex of cobalt

The statement is false. Chlorophyll is a green pigment in plants that is essential for photosynthesis. The central metal in chlorophyll is magnesium \((Mg^{2+})\), not cobalt. Magnesium is centrally coordinated in the porphyrin ring of chlorophyll and is essential for the pigment's ability to capture light energy and convert it into chemical energy.

3. Enzymes are complexes of metal ions with carboxypeptidase A and carbonic anhydrase

The statement is true.  Many enzymes are metalloproteins, which means they contain metal ions that are integral to their activity. Carboxypeptidase A is a proteolytic enzyme that requires zinc \((Zn^{2+})\) for its function. Carbonic anhydrase is another enzyme that requires zinc for catalyzing the formation of carbonic acid from carbon dioxide and water. Metal ions are essential for the catalytic activity of these enzymes.

4. Nickel is purified by Mond’s process depending on its complex forming ability with carbon monoxide

The statement is true. Mond's process is a method used to purify nickel by forming a volatile nickel carbonyl complex \(([Ni(CO)]_4)\). Nickel ore is reacted with carbon monoxide to form this complex, which is then decomposed at higher temperatures to yield pure nickel. This process relies on nickel's ability to form a stable complex with carbon monoxide.

Summary

The incorrect statement is: Chlorophyll, the green pigment in plants, is the complex of cobalt.