Question

Which among following statements is true about $\mathrm{Na_4[Fe(CN)_6]}$?

• A. The complex ion carries $-4$ charge.
• B. It is a neutral complex.
• C. The oxidation state of $\mathrm{Fe}$ in this complex is $+6$.
• D. The C.N. of $\mathrm{Fe}$ in this complex is 10.

Hint:

Always separate the alkali metal counter-ions first to determine the exact charge on a coordination sphere. For $\mathrm{Na_4[Complex]}$, the sphere is instantly assigned a $-4$ charge, whereas for $\mathrm{K_3[Complex]}$, it is assigned a $-3$ charge.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify the true chemical statement regarding the coordination compound Sodium ferrocyanide, $\mathrm{Na_4[Fe(CN)_6]}$.

Step 2: Key Formula or Approach:
We analyze the compound by separating it into its constituent counter-ions and coordination sphere: $$\mathrm{Na_4[Fe(CN)_6] \rightleftharpoons 4Na^+ + [Fe(CN)_6]^{4-}}$$ We then evaluate the oxidation state and coordination number (C.N.) of the central metal atom inside the coordination sphere.

Step 3: Detailed Explanation:
Let's analyze each statement systematically:

• Complex Ion Charge: Sodium ($\mathrm{Na}$) is an alkali metal that always carries a $+1$ charge as a cation. Since there are $4$ sodium ions outside the coordination sphere contributing a total charge of $+4$, the complex ion counter-balancing them must hold an overall charge of $-4$. Thus, the complex ion is correctly written as $\mathrm{[Fe(CN)_6]^{4-}}$. Statement (A) is completely true.

• Complex Type: The coordination sphere itself carries a net negative charge, making $\mathrm{[Fe(CN)_6]^{4-}}$ an anionic complex, not a neutral complex. Statement (B) is false.

• Oxidation State of $\mathrm{Fe}$: Let $x$ be the oxidation state of iron. Each cyanide ($\mathrm{CN^-}$) ligand carries a charge of $-1$. Setting the sum equal to the total charge of the complex ion: $$x + 6(-1) = -4 \implies x - 6 = -4 \implies x = +2$$ The oxidation state of $\mathrm{Fe}$ is $+2$, not $+6$. Statement (C) is false.

• Coordination Number (C.N.): The coordination number is defined as the total number of $\sigma$-bonds formed between the donor atoms of the ligands and the central metal atom. Since there are 6 monodentate $\mathrm{CN^-}$ ligands, the coordination number of $\mathrm{Fe}$ is 6. Statement (D) is false.

Step 4: Final Answer:
The true statement is that the complex ion carries a $-4$ charge, corresponding to option (A).