Analyzing the Complex [NiCl₄]^2-

Let's carefully examine each statement regarding the complex ion [NiCl₄]^2- to determine which ones are correct.

(A) Ni has oxidation state +2

To find the oxidation state of Nickel (Ni) in the complex [NiCl₄]^2-, we know the overall charge of the complex is -2 and the charge of each chloride ion (Cl^-) is -1.

Let the oxidation state of Ni be \(x\). The equation is:

\(x + 4 \times (-1) = -2\)

\(x - 4 = -2\)

\(x = -2 + 4\)

\(x = +2\)

So, the oxidation state of Ni in [NiCl₄]^2- is +2. Statement (A) is correct.

(B) Cl is a weak field ligand

Ligands are classified as strong field or weak field based on their position in the spectrochemical series. The spectrochemical series lists common ligands in order of increasing crystal field splitting energy. Chloride ion (Cl^-) is located towards the lower end of the spectrochemical series, making it a weak field ligand. Weak field ligands cause smaller splitting of d orbitals.

Statement (B) is correct.

(C) Compound is paramagnetic

Paramagnetism is determined by the presence of unpaired electrons. First, find the number of d electrons for the central metal ion.

• Electronic configuration of neutral Ni (Z=28): [Ar] 3d⁸ 4s².
• Ni is in the +2 oxidation state, so it loses the two 4s electrons.
• Electronic configuration of Ni²⁺: [Ar] 3d⁸. There are 8 d electrons.

The complex [NiCl₄]^2- is typically tetrahedral due to the bulky nature of chloride ligands and Ni(II) being a d⁸ ion with weak field ligands. In a tetrahedral crystal field, the five d orbitals split into a lower energy set (e) and a higher energy set (t₂).

Filling 8 electrons in tetrahedral splitting (e⁴ t₂⁴):

• The lower energy 'e' orbitals get 4 electrons: \(\uparrow\downarrow \uparrow\downarrow\) (0 unpaired)
• The higher energy 't₂' orbitals get 4 electrons: \(\uparrow\downarrow \uparrow \uparrow\) (2 unpaired)

Total number of unpaired electrons = 0 (from e) + 2 (from t₂) = 2.

Since there are unpaired electrons, the compound [NiCl₄]^2- is paramagnetic.

Statement (C) is correct.

(D) dsp2 hybridisation

dsp2 hybridisation leads to a square planar geometry. [NiCl₄]^2- with the d⁸ Ni²⁺ ion and weak field Cl^- ligands forms a tetrahedral complex. Tetrahedral complexes typically involve sp3 hybridisation.

dsp2 hybridisation occurs in d⁸ complexes with strong field ligands that cause pairing of electrons, vacating an inner d orbital (specifically \(3d_{x^2-y^2}\)) for hybridisation with 4s and two 4p orbitals. Since Cl^- is a weak field ligand, pairing is not forced, and a tetrahedral geometry is favoured with sp3 hybridisation.

Statement (D) is incorrect.

(E) Low spin complex

The terms 'low spin' and 'high spin' typically apply clearly to octahedral and sometimes square planar complexes, particularly for d⁴ to d⁷ configurations where the choice between pairing and filling higher energy orbitals depends on the relative magnitudes of crystal field splitting energy ($\Delta$) and pairing energy (P). In tetrahedral complexes, the crystal field splitting energy ($\Delta_t$) is much smaller than the pairing energy (P) ($\Delta_t \approx \frac{4}{9}\Delta_o$, and $\Delta_o$ itself might be less than P for weak field ligands). This means electrons tend to occupy orbitals singly as much as possible before pairing, characteristic of high spin complexes.

For a d⁸ configuration in a tetrahedral field, the electron distribution (e⁴ t₂⁴) naturally results in 2 unpaired electrons regardless of whether the splitting is large or small relative to pairing energy. The concept of 'low spin' (forced pairing) doesn't apply in the same way. However, if classified, tetrahedral complexes with weak field ligands are considered high spin due to the small splitting.

Statement (E) is incorrect.

Summary of Statements

Based on our analysis, statements (A), (B), and (C) are correct for the complex [NiCl₄]^2-.

Revision Table: Key Concepts for Coordination Complexes

Additional Information on Complex Properties

Understanding coordination complexes involves several key concepts:

• Coordination Number: This is the number of ligands attached to the central metal ion. In [NiCl₄]^2-, the coordination number is 4.
• Geometry: The spatial arrangement of ligands around the central metal ion. For a coordination number of 4, common geometries are tetrahedral and square planar. Tetrahedral geometry is favoured by large ligands, weak field ligands, and metal ions like Ni(II) with d⁸ configuration when interacting with weak fields. Square planar geometry is favoured by d⁸ ions with strong field ligands.
• Spectrochemical Series: This series ranks ligands based on their ability to cause d orbital splitting. Examples: I^- < Br^- < Cl^- < F^- < OH^- < H₂O < SCN^- < NH₃ < en < CN^- < CO. Ligands on the left are weak field, those on the right are strong field.
• Magnetic Moment: This is a measurable property that quantifies paramagnetism. It depends on the number of unpaired electrons. The spin-only magnetic moment (\(\mu_s\)) is given by the formula \(\mu_s = \sqrt{n(n+2)}\) Bohr Magnetons (BM), where \(n\) is the number of unpaired electrons. For [NiCl₄]^2-, with \(n=2\), \(\mu_s = \sqrt{2(2+2)} = \sqrt{8} \approx 2.83\) BM.