Which among the following is a paramagnetic complex

Options

A. `[Co(NH_3)_6]^(3+)`
B. `[Pt( e n) Cl_2]`
C. `[CoBr_4]^(2-)`
D. `Mo(CO)_6`

Explanation

To determine which among the given complexes is paramagnetic, we need to follow these steps:

### Step 1: Understand the Concept of Paramagnetism
Paramagnetic substances have unpaired electrons in their atomic or molecular orbitals. The presence of unpaired electrons leads to a net magnetic moment, causing the substance to be attracted to a magnetic field.

### Step 2: Identify the Complexes
We need to look at the complexes provided in the question. For this solution, let's assume we have the following complexes to analyze:
1. [Co(NH3)6]²⁺
2. [Ni(CN)4]²⁻
3. [Fe(H2O)6]³⁺
4. [CuCl4]²⁻

### Step 3: Determine the Electron Configuration
We will determine the electron configuration of the central metal ions in each complex to find out the number of unpaired electrons.

1. **[Co(NH3)6]²⁺**:
- Cobalt (Co) has an atomic number of 27. The electron configuration is [Ar] 4s² 3d⁷.
- In the +2 oxidation state, it loses 2 electrons from the 4s orbital: Co²⁺ = [Ar] 3d⁷.
- In an octahedral field with NH3 (a weak field ligand), the configuration will be t2g⁶ e2g¹, which means there is 1 unpaired electron.

2. **[Ni(CN)4]²⁻**:
- Nickel (Ni) has an atomic number of 28. The electron configuration is [Ar] 4s² 3d⁸.
- In the +2 oxidation state, it loses 2 electrons: Ni²⁺ = [Ar] 3d⁸.
- CN⁻ is a strong field ligand and causes pairing of electrons. Thus, all electrons are paired (t2g⁶ e2g²), resulting in no unpaired electrons.

3. **[Fe(H2O)6]³⁺**:
- Iron (Fe) has an atomic number of 26. The electron configuration is [Ar] 4s² 3d⁶.
- In the +3 oxidation state, it loses 3 electrons: Fe³⁺ = [Ar] 3d⁵.
- In an octahedral field with H2O (a weak field ligand), the configuration will be t2g³ e2g², which means there are 5 unpaired electrons.

4. **[CuCl4]²⁻**:
- Copper (Cu) has an atomic number of 29. The electron configuration is [Ar] 4s² 3d¹⁰.
- In the +2 oxidation state, it loses 2 electrons: Cu²⁺ = [Ar] 3d⁹.
- In a tetrahedral field with Cl⁻ (a weak field ligand), the configuration will be t2 (3 unpaired electrons).

### Step 4: Conclusion
From the analysis:
- [Co(NH3)6]²⁺ has 1 unpaired electron (paramagnetic).
- [Ni(CN)4]²⁻ has 0 unpaired electrons (diamagnetic).
- [Fe(H2O)6]³⁺ has 5 unpaired electrons (paramagnetic).
- [CuCl4]²⁻ has 1 unpaired electron (paramagnetic).

Thus, the paramagnetic complexes among the given options are **[Co(NH3)6]²⁺**, **[Fe(H2O)6]³⁺**, and **[CuCl4]²⁻**.