Question

The anion of acetylacetone (acac) forms $Co (acac)_3$ chelate with $Co^{3+}$. The rings of the chelate are :

• A. Five membered
• B. Four membered
• C. Six membered
• D. Three membered

Answer 1

The Correct Option is C

The problem involves determining the size of the chelate rings formed in the coordination compound $Co(acac)_3$. Acetylacetone (acac) acts as a bidentate ligand, and its deprotonated form, the acetylacetonate ion, binds to the metal center $Co^{3+}$.

Let's analyze how the chelation occurs:

1. Acetylacetone (acac), in its anion form, can donate electrons from two oxygen atoms. These oxygens are bonded to two carbonyl groups on either end of the ligand.
2. Upon coordination, each acetylacetonate ion forms two bonds with the metal center, making it a bidentate ligand. This results in the formation of a five-membered ring with the cobalt ion.
3. In the resulting coordination complex, $Co(acac)_3$, each acetylacetone forms a similar structure.

Each acac ligand forms a six-membered chelate ring with the metal ion:

• One $C=C$ bond in the acetylacetonate and the $O-C-Co$ framework contribute to the ring formation.
• Counting the atoms involved, including the metal center $Co^{3+}$, illustrates six members in the ring: C-O-Co-O-C-C.

The correct answer, based on this structure, is that the rings of the chelate are six-membered.

Option Analysis:

• Five-membered: Incorrect, as the chelate rings include the cobalt ion and additional atoms from the ligand.
• Four-membered: Incorrect, as this would not include enough atoms to complete either part of the acac ligand or the metal coordination.
• Six-membered: Correct, as properly explained above.
• Three-membered: Incorrect, such a small ring would be highly strained and is not structurally feasible here.

Therefore, the correct option is "Six membered."