Which of the following complexes formed by `Cu^(2+)` ions is most stable ?

Options

A. `Cu^(2+)+4NH_(3) rarr [Cu(NH_(3))_(4)]^(2+), log K=11.6`
B. `Cu^(2+)+4CN^(-) rarr [Cu(CN)_(4)]^(2-), log K=27.3`
C. `Cu^(2+)+2en^(-) rarr [Cu(en)_(2)]^(2+), log K=15.4`
D. `Cu^(2+)+4H_(2)O rarr [Cu(H_(2)O)_(4)]^(2+), log K =8.9`

Explanation

To determine which complex formed by `Cu^(2+)` ions is most stable, we will analyze the stability constants (log K values) provided for each complex. The stability constant indicates how stable a complex is; a higher log K value means a more stable complex.

### Step-by-Step Solution:

1. **Identify the complexes and their log K values:**
- Copper tetraammine complex: log K = 11.6
- Copper cyanide complex: log K = 27.3
- Copper ethylenediamine (en) complex: log K = 15.4
- Copper aqua complex (with water): log K = 8.9

2. **Understand the significance of log K values:**
- The stability constant (K) is a measure of the stability of a complex in solution. It is defined as the ratio of the concentration of the products (the complex) to the concentration of the reactants.
- A higher log K value indicates a greater stability of the complex.

3. **Compare the log K values:**
- Copper tetraammine: log K = 11.6
- Copper cyanide: log K = 27.3
- Copper ethylenediamine: log K = 15.4
- Copper aqua: log K = 8.9

4. **Identify the maximum log K value:**
- The maximum log K value is 27.3 for the copper cyanide complex.

5. **Conclusion:**
- Since the copper cyanide complex has the highest log K value, it is the most stable complex formed by `Cu^(2+)` ions.

### Final Answer:
The most stable complex formed by `Cu^(2+)` ions is the **copper cyanide complex** (Cu(CN)₄)²⁻ with a log K value of 27.3.