`Cu(CN)_(4)^(2-)` is colourless as it absorbs light in

Options

A. Visible region
B. Ultraviolet region
C. Infrared region
D. All above are wrong

Answer

To determine why the complex ion \( \text{Cu(CN)}_4^{3-} \) is colorless, we can analyze its electronic structure and the nature of light absorption.

### Step 1: Identify the oxidation state of copper in the complex
In the complex \( \text{Cu(CN)}_4^{3-} \), the cyanide ion \( \text{CN}^- \) has a charge of -1. Since there are four cyanide ions, the total negative charge contributed by them is -4. The overall charge of the complex is -3. Therefore, the oxidation state of copper can be calculated as follows:

\[ \text{Oxidation state of Cu} + 4(-1) = -3 \]
\[ \text{Oxidation state of Cu} - 4 = -3 \]
\[ \text{Oxidation state of Cu} = +1 \]

### Step 2: Determine the electronic configuration of copper in this oxidation state
Copper (Cu) has an atomic number of 29. The electronic configuration of neutral copper is:

\[ \text{Cu: } [\text{Ar}] 4s^1 3d^{10} \]

In the +1 oxidation state, copper loses one electron from the 4s orbital:

\[ \text{Cu}^+: [\text{Ar}] 3d^{10} \]

### Step 3: Analyze the d-orbital electron configuration
In the \( \text{Cu}^+ \) state, the d-orbitals are fully filled with 10 electrons (3d10). A fully filled d-orbital means there are no unpaired electrons.

### Step 4: Understand the implications for color
Color in transition metal complexes is often due to the presence of unpaired electrons in the d-orbitals, which can absorb visible light to promote an electron to a higher energy level. However, since \( \text{Cu(CN)}_4^{3-} \) has a fully filled d-orbital (3d10), it does not have any unpaired electrons to absorb visible light.

### Step 5: Conclusion on light absorption
Since \( \text{Cu(CN)}_4^{3-} \) does not absorb visible light due to the absence of unpaired electrons, it appears colorless. It may absorb light in the ultraviolet (UV) or infrared (IR) regions, but not in the visible region.

### Final Answer
Thus, \( \text{Cu(CN)}_4^{3-} \) is colorless as it absorbs light in the ultraviolet (UV) or infrared (IR) regions, but not in the visible region.