The correct increasing order of trans-effect of the following species is

Options

A. `NH_(3)gtCN^(-)gtBr^(-)gtC_(6)H_(5)^(-)`
B. `CN^(-)gtC_(6)H_(5)^(-)gtBr^(-)gtNH_(3)`
C. `Br^(-)gtCN^(-)gtNH_(3)gtC_(6)H_(5)^(-)`
D. `CN^(-)gtBr^(-)gtC_(6)H_(5)^(-)gtNH_(3)`

Answer

To determine the correct increasing order of trans-effect for the given species, we need to follow these steps:

### Step 1: Understand the Concept of Trans-Effect
The trans-effect refers to the influence of a ligand on the substitution of another ligand that is positioned trans (opposite) to it in a coordination complex. The strength of the trans-effect varies among different ligands.

### Step 2: Identify the Ligands
The ligands mentioned in the question are:
- CN⁻ (cyanide)
- C₆H₅⁻ (phenyl)
- Br⁻ (bromide)
- NH₃ (ammonia)

### Step 3: Rank the Ligands Based on Their Trans-Effect
From the knowledge of trans-effect, we can rank the ligands based on their ability to facilitate the substitution of the trans ligand:

1. **CN⁻** is a strong field ligand and has a strong trans-effect.
2. **C₆H₅⁻ (phenyl)** is weaker than CN⁻ but stronger than Br⁻.
3. **Br⁻** is a weak field ligand and has a weaker trans-effect compared to both CN⁻ and C₆H₅⁻.
4. **NH₃** is also a weak field ligand but weaker than Br⁻ in terms of trans-effect.

### Step 4: Establish the Increasing Order
Based on the strength of the trans-effect, we can establish the following increasing order:
- NH₃ < Br⁻ < C₆H₅⁻ < CN⁻

### Step 5: Conclusion
Thus, the correct increasing order of trans-effect for the given species is:
**NH₃ < Br⁻ < C₆H₅⁻ < CN⁻**