`[Mn(CO)_(4)NO]` is diamagnetic because:

Options

A. Mn metal is diamagnetiic in free state
B. Mn is in +1 oxidation state in this complex
C. No is present as positive ligand
D. All of these

Explanation

To determine why the complex `[Mn(CO)₄NO]` is diamagnetic, we need to analyze the oxidation state of manganese (Mn) in the complex and the electronic configurations of the involved species.

### Step-by-Step Solution:

**Step 1: Determine the oxidation state of Mn in the complex.**
- The complex is `[Mn(CO)₄NO]`.
- Carbon monoxide (CO) is a neutral ligand, contributing 0 to the oxidation state.
- The nitrosyl (NO) ligand can be considered as neutral or as a negative ligand depending on its bonding. In this case, we will consider it as a neutral ligand for simplicity.
- Let the oxidation state of Mn be \( x \).
- The overall charge of the complex is neutral, so:
\[ x + 0 + 0 = 0 \implies x = 0 \]
- Thus, Mn is in the 0 oxidation state.

**Step 2: Determine the electron configuration of Mn in the 0 oxidation state.**
- The atomic number of Mn is 25, so its electron configuration is:
\[ [Ar] 4s^2 3d^5 \]

**Step 3: Analyze the effect of ligands on the electron configuration.**
- CO is a strong field ligand and will cause pairing of the electrons in the d-orbitals.
- In the presence of 4 CO ligands, the configuration will change due to pairing:
- The 4s orbital will lose its electrons first, and the 3d electrons will pair up.
- The new configuration will be:
\[ 3d^6 \quad (all \, paired) \]

**Step 4: Determine the magnetic properties of the complex.**
- A species is diamagnetic if all its electrons are paired.
- Since the new electron configuration of Mn in the complex is \( 3d^6 \) with all paired electrons, the complex `[Mn(CO)₄NO]` is diamagnetic.

**Step 5: Verify the electronic configuration of CO and NO.**
- CO is neutral and has a total of 14 electrons (6 from C and 8 from O), and it is also diamagnetic.
- For NO, when considered as a neutral ligand, it has 15 electrons (7 from N and 8 from O), which makes it paramagnetic. However, in this complex, it behaves as a neutral ligand contributing to the overall diamagnetism.

### Conclusion:
The complex `[Mn(CO)₄NO]` is diamagnetic because:
- Mn is in the 0 oxidation state and has a \( 3d^6 \) configuration with all paired electrons due to the strong field ligand CO.