Question

Vitamin $B_{12}$ is a complex compound of :

• A. $Fe^{2+}$
• B. $Mg^{2+}$
• C. $Co^{3+}$
• D. $Cu^{2+}$

Hint:

Match iron with haemoglobin and magnesium with chlorophyll first. The remaining key clue is that vitamin $B_{12}$ is a cobalamin with a cobalt centre.

Answer 1

The Correct Option is C

Step 1: Concept
Many crucial biological molecules are coordination complexes consisting of a specific central metal ion bound by a large organic macrocycle.

Step 2: Meaning
Vitamin $B_{12}$ has a biochemical name that hints at its central metal atom: cyanocobalamin.

Step 3: Analysis
Examining the options, $Fe^{2+}$ is found in hemoglobin and myoglobin. $Mg^{2+}$ is the central metal ion in chlorophyll. The name "cobalamin" points directly to the element Cobalt.

Step 4: Conclusion
Vitamin $B_{12}$ is a coordination complex containing a Cobalt ion ($Co^{3+}$) at its core, surrounded by a corrin ring.

Final Answer: (C)

Answer 2

Concept:

• Match each metal ion with its well-known biological coordination compound.
• Vitamin $B_{12}$ contains a corrin-ring metal centre rather than the porphyrin centres found in haemoglobin or chlorophyll.

Step 1: Eliminate the common biological distractors.
$Fe^{2+}$ is associated with the haem group in haemoglobin.
$Mg^{2+}$ is the central ion in chlorophyll, while $Cu^{2+}$ occurs in several copper enzymes.

Step 2: Identify the corrin complex.
Vitamin $B_{12}$ is a cobalamin. Its corrin ring coordinates a cobalt ion at the centre.

Step 3: Match the required oxidation state.
In cyanocobalamin, the cobalt centre is represented as cobalt in the $+3$ oxidation state.
Therefore, the matching ion is $Co^{3+}$.

Final Answer: $Co^{3+}$, option C.