Crystal field theory provides correct electronic distribution of central metal under surrounding ligannd field, hence it clearly explains magnetic moment, colour of a complex.
Q. Which of the following complex is high spin?

Options

A. `K_(4)[Fe(CN)_(6)]^(-)`
B. `[PtCl_(4)]^(2-)`
C. `[CoF_(6)]^(3-)`
D. `[Ni(NH_(3))_(6)]^(2+)`

Explanation

To determine which of the given complexes is a high spin complex, we need to understand the concept of crystal field theory and the behavior of ligands in relation to the central metal ion.

### Step-by-Step Solution:

1. **Understand High Spin Complexes**:
- High spin complexes are formed by weak field ligands. In these complexes, the pairing energy of electrons is greater than the energy of orbital splitting. As a result, electrons do not pair up in the lower energy orbitals and occupy higher energy orbitals instead, leading to a larger number of unpaired electrons.

**Hint**: Remember that high spin complexes have more unpaired electrons due to weak field ligands.

2. **Identify the Ligands in Each Complex**:
- Analyze the ligands present in each of the options provided. Ligands can be classified as strong field or weak field based on their ability to cause splitting of the d-orbitals.

3. **Evaluate Each Complex**:
- **Option 1**: Contains cyanide (CN⁻), which is a strong field ligand. Strong field ligands lead to low spin complexes.
- **Option 2**: Contains chlorine (Cl⁻) with platinum. Chlorine behaves as a weak field ligand with 3D elements, but as a strong field ligand with 4D and 5D elements. Here, it is likely a strong field ligand due to platinum being a 4D element.
- **Option 3**: Contains fluorine (F⁻). Fluorine behaves as a weak field ligand with 3D elements, leading to the formation of high spin complexes.
- **Option 4**: Contains ammonia (NH₃). Ammonia can behave as a weak field ligand, particularly when coordinated with a metal in a +2 oxidation state and a coordination number of 6, leading to high spin complexes.

4. **Conclusion**:
- Based on the analysis, the complexes that are high spin are:
- **Option 3** (Fluorine as a weak field ligand)
- **Option 4** (Ammonia behaving as a weak field ligand under specific conditions).

**Final Answer**: The complexes that are high spin are **Option 3 and Option 4**.