There are four complexes of Ni. Select the complex(es) which will be attracted by magnetic field:
(I) `[Ni(CN)_4]^(2-)`
(II) `[NiCl_(4)]^(2-)`
(III) `Ni(CO_(4))`
(IV) `[Ni(H_(2)O)_(6)]^(2+)`

Options

A. I only
B. IV only
C. II, III and IV
D. II and IV

Answer

To determine which of the given nickel complexes are attracted by a magnetic field, we need to analyze the magnetic properties of each complex based on the presence of unpaired electrons. A complex is paramagnetic (and thus attracted to a magnetic field) if it has unpaired electrons, while it is diamagnetic (and not attracted) if all electrons are paired.

### Step-by-Step Solution:

1. **Identify the complexes:**
- (I) `[Ni(CN)_4]^(2-)`
- (II) `[NiCl_(4)]^(2-)`
- (III) `Ni(CO_(4))`
- (IV) `[Ni(H_(2)O)_(6)]^(2+)`

2. **Determine the oxidation state of Ni in each complex:**
- For (I) `[Ni(CN)_4]^(2-)`:
- Let the oxidation state of Ni be x.
- CN is a -1 ligand, so:
\[ x + 4(-1) = -2 \]
\[ x - 4 = -2 \]
\[ x = +2 \]
- For (II) `[NiCl_(4)]^(2-)`:
- Let the oxidation state of Ni be x.
- Cl is a -1 ligand, so:
\[ x + 4(-1) = -2 \]
\[ x - 4 = -2 \]
\[ x = +2 \]
- For (III) `Ni(CO_(4))`:
- Let the oxidation state of Ni be x.
- CO is a neutral ligand, so:
\[ x + 0 = 0 \]
\[ x = 0 \]
- For (IV) `[Ni(H_(2)O)_(6)]^(2+)`:
- Let the oxidation state of Ni be x.
- H2O is a neutral ligand, so:
\[ x + 0 = +2 \]
\[ x = +2 \]

3. **Determine the electron configuration of Ni in each oxidation state:**
- For Ni in +2 state:
- Configuration: Argon (Ar) 4s² 3d⁸ → 4s⁰ 3d⁸
- For Ni in 0 state:
- Configuration: Argon (Ar) 4s² 3d⁸

4. **Analyze the ligand field strength:**
- (I) `[Ni(CN)_4]^(2-)`: CN is a strong field ligand, which causes pairing of electrons in the 3d orbitals.
- Result: All electrons are paired (Diamagnetic).
- (II) `[NiCl_(4)]^(2-)`: Cl is a weak field ligand, which does not cause pairing.
- Result: 3d⁸ configuration has 2 unpaired electrons (Paramagnetic).
- (III) `Ni(CO_(4))`: CO is a strong field ligand, which causes pairing.
- Result: All electrons are paired (Diamagnetic).
- (IV) `[Ni(H_(2)O)_(6)]^(2+)`: H2O is a weak field ligand, which does not cause pairing.
- Result: 3d⁸ configuration has 2 unpaired electrons (Paramagnetic).

5. **Conclusion:**
- The complexes that are attracted by a magnetic field (paramagnetic) are:
- (II) `[NiCl_(4)]^(2-)`
- (IV) `[Ni(H_(2)O)_(6)]^(2+)`

### Final Answer:
The complexes that will be attracted by a magnetic field are (II) `[NiCl_(4)]^(2-)` and (IV) `[Ni(H_(2)O)_(6)]^(2+)`.