Which of the following statement is not true for the reaction given below:
`[Cu(H_(2)O)_(4)]^(2+)+ 4NH_(4) hArr [Cu(NH_(3))_(4)]^(2+)+4H_(2)O`

Options

A. It is ligand substitution reaction
B. `NH_(3)` is a relatively strong field ligand while `H_(2)O` is a weak field ligand
C. During the reaction, there is a change in colour from light blue to dark blue
D. `[Cu(NH_(3))_(4)]^(2+)`, has a tetrahedral structure and is paramagnetic

Explanation

To determine which statement is not true for the given reaction:

\[ [Cu(H_2O)_4]^{2+} + 4NH_3 \rightleftharpoons [Cu(NH_3)_4]^{2+} + 4H_2O \]

we will analyze each statement provided.

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
- The reaction involves the substitution of ligands in the complex ion. The water ligands in \([Cu(H_2O)_4]^{2+}\) are replaced by ammonia ligands to form \([Cu(NH_3)_4]^{2+}\).
- **Conclusion**: This is a ligand substitution reaction.

2. **Evaluate the Strength of Ligands**:
- According to Crystal Field Theory (CFT), ammonia (NH3) is a stronger field ligand compared to water (H2O). This is because NH3 has a higher tendency to donate electron pairs due to its electronegativity and molecular structure.
- **Conclusion**: The statement that NH3 is a relatively strong field ligand while H2O is a weak field ligand is true.

3. **Analyze the Color Change**:
- The complex \([Cu(H_2O)_4]^{2+}\) is typically light blue, while \([Cu(NH_3)_4]^{2+}\) is dark blue. The change in ligands affects the crystal field splitting, leading to a change in color.
- **Conclusion**: The statement about the color change from light blue to dark blue is correct.

4. **Determine the Geometry and Magnetic Properties of \([Cu(NH_3)_4]^{2+}\)**:
- Copper in the +2 oxidation state has a d9 electronic configuration. In the presence of strong field ligands like NH3, the electrons will pair up, leading to a square planar geometry rather than tetrahedral.
- The square planar geometry is associated with dsp2 hybridization, and since there are no unpaired electrons in the d-orbitals, the complex is actually diamagnetic.
- **Conclusion**: The statement that \([Cu(NH_3)_4]^{2+}\) has a tetrahedral structure and is paramagnetic is false.

### Final Answer:
The statement that is not true for the reaction is:
- **Option 4**: "Copper \([Cu(NH_3)_4]^{2+}\) has a tetrahedral structure and it is paramagnetic."