Which of the following complex is example of strongest reducing agent ?

Options

A. `[Co(CN)_(6)]^(-3)`
B. `[Co(CN)_(6)]^(-4)`
C. `[Ag(CN)_(2)]^(-1)`
D. `[Cu(CN)_(4)]^(-3)`

Answer

To determine which of the given complexes is the strongest reducing agent, we will analyze the oxidation states and electronic configurations of the central metal ions in each complex. A reducing agent is a species that donates electrons and gets oxidized in the process.

### Step-by-Step Solution:

1. **Identify the Central Metal Ions and Their Oxidation States:**
- Let's denote the complexes as follows:
- Option A: Cobalt (Co) in +3 oxidation state
- Option B: Cobalt (Co) in +2 oxidation state
- Option C: Silver (Ag) in +1 oxidation state
- Option D: Copper (Cu) in +1 oxidation state

2. **Determine the Atomic Numbers and Electronic Configurations:**
- Cobalt (Co): Atomic number = 27
- Electronic configuration: [Ar] 4s² 3d⁷
- Co³⁺: [Ar] 3d⁶ (loses 3 electrons)
- Cobalt (Co): Atomic number = 27
- Co²⁺: [Ar] 3d⁷ (loses 2 electrons)
- Silver (Ag): Atomic number = 47
- Electronic configuration: [Kr] 4d¹⁰ 5s¹
- Ag⁺: [Kr] 4d¹⁰ (loses 1 electron)
- Copper (Cu): Atomic number = 29
- Electronic configuration: [Ar] 4s¹ 3d¹⁰
- Cu⁺: [Ar] 3d¹⁰ (loses 1 electron)

3. **Analyze the Stability and Ability to Lose Electrons:**
- **Cobalt (Co³⁺)**:
- Configuration: 3d⁶. It can easily lose one more electron to achieve a half-filled d orbital (3d⁵), which is stable. Thus, it can act as a strong reducing agent.
- **Cobalt (Co²⁺)**:
- Configuration: 3d⁷. It would need to lose two electrons to reach a stable state, which is less favorable energetically, making it a weaker reducing agent compared to Co³⁺.
- **Silver (Ag⁺)**:
- Configuration: 4d¹⁰. It has a fully filled d orbital and is stable, so it does not tend to lose electrons, making it a poor reducing agent.
- **Copper (Cu⁺)**:
- Configuration: 3d¹⁰. Similar to silver, it has a fully filled d orbital and does not easily lose electrons, thus it is also a poor reducing agent.

4. **Conclusion:**
- Among the given options, **Cobalt in the +3 oxidation state (Co³⁺)** is the strongest reducing agent because it can easily lose an electron to achieve a more stable half-filled d orbital configuration.

### Final Answer:
The strongest reducing agent among the given complexes is **Option A: Cobalt (Co) in +3 oxidation state**.