The IUPAC name of the coordination compound K3[Fe(CN)6] is
(a) tripotassium hexacyanoiron (II)
(b) p

Q: Why isn't the name 'tripotassium hexacyanoferrate'?

In IUPAC nomenclature, we do not use prefixes (like di, tri, tetra) for the counter-ions outside the coordination sphere. The oxidation state of the metal and the charge of the ligands inherently tell a chemist how many potassium ions are needed to balance the charge.

Q: Why do we use 'ferrate' instead of 'iron'?

When the coordination sphere (the part inside the square brackets) has a net negative charge, it is an anionic complex. IUPAC rules state that the central metal in an anionic complex must end with the suffix '-ate'. For iron, we use its Latin root 'ferrum', making it 'ferrate'.

Q: How did we know the charge on the complex anion is -3?

Potassium is an alkali metal in Group 1, so it always forms a $+1$ ion. Since there are three potassium ions ($K_3$), the total positive charge is $+3$. To make the compound electrically neutral, the complex anion must have a balancing charge of $-3$.

Q: What is the charge on the cyanide ligand?

The cyanide ligand ($CN^-$) is a well-known anionic ligand that carries a charge of $-1$.

Q: What would the name be if the formula was $K_4[Fe(CN)_6]$?

If it were $K_4[Fe(CN)_6]$, the complex anion would be $[Fe(CN)_6]^{4-}$. Calculating the oxidation state: $x + 6(-1) = -4$, which gives $x = +2$. The name would then be potassium hexacyanoferrate(II).

Question

The IUPAC name of the coordination compound $K_3[Fe(CN)_6]$ is

Options:

(A) tripotassium hexacyanoiron (II)

(B) potassium hexacyanoiron (II)

(C) potassium hexacyanoferrate (III)

(D) potassium hexacyanoferrate (II)

Sigma Percentile · Accepted Answer

### Unlocking the Name: The Art of IUPAC Nomenclature for Coordination Compounds

Coordination compounds often look like a terrifying jumble of brackets, letters, and numbers. But just like a well-structured sentence, their names follow a strict, logical grammar. Let's decode the IUPAC name for the classic compound $K_3[Fe(CN)_6]$.

### Breaking Down the Complex

Imagine dropping a crystal of $K_3[Fe(CN)_6]$ into water. It doesn't shatter into individual atoms; instead, it elegantly splits into two distinct parts: the simple cations and the complex anion.

$$K_3[Fe(CN)_6] \rightarrow 3K^+ + [Fe(CN)_6]^{3-}$$

We have three potassium ions ($K^+$) acting as the counter-ions, and one bulky hexacyanoferrate ion ($[Fe(CN)_6]^{3-}$) acting as the coordination sphere.

### The Rules of the Game

According to IUPAC nomenclature rules, we must always name the **cation first**, followed by the anion.

Here is a crucial trap where many students fall: **we never use prefixes like 'tri' or 'tetra' for the simple counter-ions outside the coordination sphere.** The charge of the complex anion inherently dictates how many potassium ions are needed to balance it. Therefore, the name simply starts with **Potassium**, not 'tripotassium'.

Next, we look at the complex anion. Because the coordination sphere carries a negative charge ($-3$), the central metal cannot just be called 'iron'. It must take its Latin root and end with the suffix '-ate'. Thus, 'iron' becomes **ferrate**.

### The Math: Finding the Oxidation State

To complete the name, we must determine the oxidation state of the central iron atom. This is a simple balancing act of charges.

Let the oxidation state of iron be $x$. We know that each cyanide ($CN^-$) ligand carries a charge of $-1$. Since there are six cyanide ligands, their total charge is $-6$. The net charge on the entire complex anion is $-3$.

Setting up the equation:

$$x + 6(-1) = -3$$

$$x - 6 = -3$$

$$x = +3$$

Iron is sitting comfortably in the $+3$ oxidation state. In IUPAC nomenclature, this is written in Roman numerals in parentheses immediately after the metal's name: **(III)**.

### The Grand Finale

Now, we assemble the pieces like a puzzle:
1. **Cation:** Potassium
2. **Ligands:** Six cyanides become 'hexacyano'
3. **Metal:** Iron in an anionic complex becomes 'ferrate'
4. **Oxidation State:** (III)

Stringing them together, we get the beautiful, systematic name: **potassium hexacyanoferrate(III)**.

Once you understand the grammar, naming coordination compounds becomes less of a memory game and more of a logical puzzle!

Animated Solution for Chemistry - Coordination Compounds: The IUPAC name of the coordination compound $K_{3} [F e (C N)_{6}]$ is

Select Answer:

Visualized Solution

$K_{3} [F e (C N)_{6}] \to 3 K^{+} + [F e (C N)_{6}]^{3 -}$

\text{IUPAC Nomenclature Rules}

\text{Oxidation State Setup}

\text{Calculating Oxidation State}

\text{Final IUPAC Name}

\text{Food for Thought}

The Sigma Insight: Nomenclature, Isomerism, Importance and Werner's Theory

Unlocking the Name

Breaking Down the Complex

The Rules of the Game

The Math

The Grand Finale

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Animated Solution for Chemistry - Coordination Compounds: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The IUPAC name of the coordination compound K3​[Fe(CN)6​] is