Which of the following are paramagnetic?

A.   [NiCl₄]^2–

B.    Ni(CO)₄

C.    [Ni(CN)₄]^2–

D.    [Ni(H₂O)₆]²⁺

E.     Ni(PPh₃)₄

Choose the correct answer from the options given below :

To decide which nickel complexes are paramagnetic we examine the oxidation state of Ni, the number of d‑electrons, the geometry of the complex and whether the ligands are strong or weak field.

Complex A – [NiCl₄]^2‑

The four chloride ligands each carry a –1 charge. The overall charge of the ion is –2, therefore the nickel oxidation state is +2:

Ni²⁺ is a d⁸ metal ion. Chloride is a weak‑field ligand, so the complex adopts a tetrahedral geometry. In a tetrahedral field the splitting is small, and the electron configuration of a d⁸ ion is:

e (lower) = 4 electrons (paired) t₂ (higher) = 4 electrons → 2 unpaired electrons.

Thus the complex has two unpaired electrons and is **paramagnetic**.

Complex B – Ni(CO)₄

Carbon monoxide is a strong‑field ligand and the complex is neutral, so nickel is in the 0 oxidation state:

Ni⁰ is a d¹⁰ ion. All ten d‑electrons are paired regardless of geometry (tetrahedral), so the complex is **diamagnetic**.

Complex C – [Ni(CN)₄]^2‑

Each cyanide ligand is –1. The 2‑ charge gives nickel an oxidation state of +2:

Ni²⁺ (d⁸) with the strong‑field CN⁻ ligand prefers a square‑planar geometry. In a square‑planar field the d⁸ configuration is low‑spin, giving a completely paired set of electrons. Hence the complex is **diamagnetic**.

Complex D – [Ni(H₂O)₆]²⁺

Water is a neutral ligand; the overall 2+ charge means nickel is Ni²⁺ (d⁸). Water is a weak‑field ligand, so the complex is octahedral and high‑spin. The octahedral d⁸ configuration is:

t_2g = 6 electrons (fully paired) e_g = 2 electrons (one in each e_g orbital) → 2 unpaired electrons.

Therefore the complex is **paramagnetic**.

Complex E – Ni(PPh₃)₄

The phosphine ligand is neutral; the complex is neutral, giving Ni in the 0 oxidation state (d¹⁰). Although PPh₃ is a strong‑field ligand, a d¹⁰ configuration has all electrons paired, so the complex is **diamagnetic**.

Combining the results, the only paramagnetic complexes are **A** and **D**.