Question

What is the coordination number of the central metal ion in $[Fe(C_2O_4)_3]^{3-}$?

• A. 3
• B. 5
• C. 4
• D. 6

Hint:

Chemistry Tip: Always check the denticity of common ligands! Amine ($NH_3$) and water ($H_2O$) are monodentate (1), while oxalate ($ox$) and ethylenediamine ($en$) are bidentate (2), and EDTA is hexadentate (6).

Answer 1

The Correct Option is D

Concept: Chemistry (Coordination Compounds) - Coordination Number and Denticity.

Step 1: Identify the central metal ion and the ligands. In the given complex ion $[Fe(C_{2}O_{4})_{3}]^{3-}$, the central transition metal ion is Iron ($Fe^{3+}$). The species attached to the central metal are the ligands, which in this case are oxalate ions ($C_{2}O_{4}^{2-}$).

Step 2: Determine the total number of ligands present. Looking at the subscript outside the parentheses of the ligand in the chemical formula $[Fe(C_{2}O_{4})_{3}]^{3-}$, we can see that there are exactly 3 oxalate ligands bound to the central iron ion.

Step 3: Identify the denticity of the oxalate ligand. The coordination number is not simply the number of ligands, but the number of coordinate bonds formed with the central metal. We must determine the "denticity" (number of donor atoms) of the oxalate ion. The oxalate ion ($C_{2}O_{4}^{2-}$) has two negatively charged oxygen atoms that can simultaneously donate electron pairs to the metal. Therefore, it is a bidentate ligand.

Step 4: State the formula for calculating the coordination number. The overall coordination number of a complex is calculated by multiplying the number of ligands by their respective denticity: Coordination Number = (Number of ligands) $\times$ (Denticity).

Step 5: Calculate the final coordination number. We have 3 bidentate oxalate ligands. Substituting these values into our formula gives: Coordination Number = $3 \text{ ligands} \times 2 \text{ coordinate bonds per ligand}$. Multiplying these together yields a total of 6 coordinate covalent bonds surrounding the central Iron ion. This corresponds to an octahedral coordination geometry. $$ \therefore \text{The coordination number of the central metal ion is } 6. $$