Which of the following complex is octahedral, diamagnetic and the most stable?

Options

A. `[Ni(NH_(3))_(6)]Cl_(2)`
B. `K_(3)[Co(CN)_(6)]`
C. `Na_(3)[CoCl_(6)]`
D. `[Co(H_(2)O)_(6)]Cl_(2)`

Answer

To solve the question regarding which complex is octahedral, diamagnetic, and the most stable, we can follow these steps:

### Step 1: Understand the requirements
The question asks for a complex that is:
- **Octahedral**: This means it has six ligands surrounding the central metal ion.
- **Diamagnetic**: This means all electrons are paired, indicating that there are no unpaired electrons in the complex.
- **Most stable**: Stability is often influenced by the strength of the ligands surrounding the metal ion.

### Step 2: Analyze the options
Assume the options provided are:
1. [Cr(H2O)6]3+
2. [Co(NH3)6]3+
3. [Fe(CN)6]3-
4. [NiCl4]2-

Since all complexes are octahedral (as they have six ligands), we will focus on determining which of these complexes is diamagnetic and the most stable.

### Step 3: Determine the ligand field strength
Referring to the spectrochemical series, we can rank the ligands based on their field strength:
- CN⁻ (strong field ligand)
- NH3 (moderate field ligand)
- H2O (weak field ligand)
- Cl⁻ (very weak field ligand)

### Step 4: Check for diamagnetism
- **[Cr(H2O)6]3+**: Cr³⁺ has the electron configuration of [Ar] 3d³, which has unpaired electrons, making it paramagnetic.
- **[Co(NH3)6]3+**: Co³⁺ has the electron configuration of [Ar] 3d⁶. In the presence of NH3 (a strong field ligand), it can pair up all electrons, making it diamagnetic.
- **[Fe(CN)6]3-**: Fe³⁺ has the electron configuration of [Ar] 3d⁵. In the presence of CN⁻ (a very strong field ligand), it can also pair up all electrons, making it diamagnetic.
- **[NiCl4]2-**: Ni²⁺ has the electron configuration of [Ar] 3d⁸. However, Cl⁻ is a weak field ligand, and it does not promote pairing, resulting in unpaired electrons, making it paramagnetic.

### Step 5: Determine stability
Among the diamagnetic complexes:
- **[Co(NH3)6]3+** is stable but not as stable as [Fe(CN)6]3-.
- **[Fe(CN)6]3-** is the most stable due to the presence of CN⁻, which is a very strong field ligand.

### Conclusion
The complex that is octahedral, diamagnetic, and the most stable is **[Fe(CN)6]3-**.