The geometry and magnetic behaviour of the complex `[Ni(CO)_4]` are
(a)square planar geometry and paramagnetic
(b)tetrahedral geometry and diamagnetic
(c)square planar geometry and diamagnetic
(d)tetrahedral geometry and paramagnetic

Options

A. square planar geometry and paramagnetic
B. tetrahedral geometry and diamagnetic
C. square planar geometry and diamagnetic
D. tetrahedral geometry and paramagnetic

Explanation

To determine the geometry and magnetic behavior of the complex \([Ni(CO)_4]\), we can follow these steps:

### Step 1: Identify the oxidation state of Nickel
- In the complex \([Ni(CO)_4]\), carbon monoxide (CO) is a neutral ligand. Therefore, the oxidation state of nickel (Ni) in this complex is 0.

### Step 2: Determine the electron configuration of Nickel
- The atomic number of nickel (Ni) is 28. Its electron configuration in the ground state is:
\[ [Ar] 3d^8 4s^2 \]
- This means that nickel has 8 electrons in the 3d subshell and 2 electrons in the 4s subshell.

### Step 3: Analyze the ligand field strength
- CO is a strong field ligand, which means it has the ability to cause pairing of electrons in the d-orbitals.

### Step 4: Pairing of electrons
- Due to the strong field nature of CO, the 4s electrons will pair with the 3d electrons. The new electron configuration after pairing will be:
\[ 3d^{10} 4s^0 \]
- This indicates that all 3d orbitals are fully filled, and there are no unpaired electrons.

### Step 5: Determine the hybridization
- The hybridization of the complex can be determined by the number of ligands. Since there are four CO ligands, the hybridization will be \(sp^3\).

### Step 6: Determine the geometry
- The \(sp^3\) hybridization corresponds to a tetrahedral geometry.

### Step 7: Determine the magnetic behavior
- Since there are no unpaired electrons in the configuration \(3d^{10}\), the complex is diamagnetic.

### Conclusion
Based on the analysis, the geometry and magnetic behavior of the complex \([Ni(CO)_4]\) is:
- **Tetrahedral geometry and diamagnetic.**

Thus, the correct answer is **(b) tetrahedral geometry and diamagnetic.**

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