To determine which of the given nickel complexes are attracted by a magnetic field, we need to analyze the oxidation state of nickel in each complex and the nature of the ligands involved. The presence of unpaired electrons in the d-orbitals of the nickel ion will dictate whether the complex is paramagnetic (attracted to a magnetic field) or diamagnetic (not attracted).
### Step-by-Step Solution:
1. **Identify the oxidation state of Nickel in each complex:**
- For `[Ni(CN)₄]²⁻`:
- Let the oxidation state of Ni be \( x \).
- The charge of CN is -1, and there are 4 CN ligands, giving a total charge of -4.
- The overall charge of the complex is -2.
- Therefore, \( x - 4 = -2 \) → \( x = +2 \).
- For `[NiCl₄]²⁻`:
- Let the oxidation state of Ni be \( x \).
- The charge of Cl is -1, and there are 4 Cl ligands, giving a total charge of -4.
- The overall charge of the complex is -2.
- Therefore, \( x - 4 = -2 \) → \( x = +2 \).
- For `[Ni(CO)₄]`:
- Let the oxidation state of Ni be \( x \).
- CO is a neutral ligand, so it contributes 0 to the charge.
- The overall charge of the complex is 0.
- Therefore, \( x = 0 \).
- For `[Ni(NH₃)₆]²⁺`:
- Let the oxidation state of Ni be \( x \).
- NH₃ is a neutral ligand, contributing 0 to the charge.
- The overall charge of the complex is +2.
- Therefore, \( x = +2 \).
2. **Determine the electronic configuration of Ni in each complex:**
- The atomic number of Ni is 28, and its ground state electronic configuration is \( [Ar] 3d^8 4s^2 \).
- In the +2 oxidation state, Ni loses two electrons from the 4s orbital, resulting in \( 3d^8 \).
3. **Analyze the ligand strength and electron pairing:**
- **For `[Ni(CN)₄]²⁻`:**
- CN⁻ is a strong field ligand, which causes pairing of electrons.
- The \( 3d^8 \) configuration will have all electrons paired, resulting in 0 unpaired electrons (diamagnetic).
- **For `[NiCl₄]²⁻`:**
- Cl⁻ is a weak field ligand, which does not cause pairing.
- The \( 3d^8 \) configuration will have 2 unpaired electrons (paramagnetic).
- **For `[Ni(CO)₄]`:**
- CO is a strong field ligand, which causes pairing of electrons.
- The \( 3d^8 \) configuration will have all electrons paired, resulting in 0 unpaired electrons (diamagnetic).
- **For `[Ni(NH₃)₆]²⁺`:**
- NH₃ is a strong field ligand, which can cause pairing.
- The \( 3d^8 \) configuration will have 2 unpaired electrons (paramagnetic).
4. **Conclusion:**
- The complexes that are attracted by a magnetic field (paramagnetic) are:
- `[NiCl₄]²⁻` (2 unpaired electrons)
- `[Ni(NH₃)₆]²⁺` (2 unpaired electrons)
Thus, the complexes that will be attracted by the magnetic field are **(II) `[NiCl₄]²⁻` and (IV) `[Ni(NH₃)₆]²⁺`.**
