Question

The complex ion NiCl \(_{4}^{2-}\) has two unpaired electrons, whereas \(\mathrm{Ni}(\mathrm{CN})_{4}^{2-}\) is diamagnetic. Propose structures for these two complex ions.

Short answer

The structures of NiCl₄²⁻ and Ni(CN)₄²⁻ are both tetrahedral with Ni²⁺ as the central atom. NiCl₄²⁻ has two unpaired electrons due to weak field ligand Cl⁻, while Ni(CN)₄²⁻ is diamagnetic with no unpaired electrons due to strong field ligand CN⁻. The d-electrons fill the orbitals differently in each complex ion, resulting in the observed differences in magnetism.

Step-by-step solution

Determine the d-electron configuration of Ni²⁺

First, we need to find out the electron configuration of the central metal ion, Ni²⁺. The Nickel (Ni) atom has 28 electrons, but since it loses 2 electrons to form Ni²⁺, it now has 26 electrons. The electron configuration for Nickel (Ni) is [Ar] 3d⁸4s², therefore for Ni²⁺ it becomes [Ar] 3d⁸ (since 2 electrons are removed from 4s shell).

Identify the Ligand Field Chemistry of Cl⁻ and CN⁻

The two ligands, Cl⁻ and CN⁻, differ in their ligand field strength: Cl⁻ is a weak field ligand and CN⁻ is a strong field ligand. Weak field ligands tend to not create a large energy gap between the d-orbitals, while strong field ligands create a large split between the d-orbitals.

Construct the Crystal Field Splitting Diagram of NiCl₄²⁻

Since Cl⁻ is a weak field ligand, the crystal field splitting diagram for a tetrahedral complex would look like this: 1. There would be no splitting in the 4s and 4p orbitals. 2. The d-orbitals will be split into a lower energy set (dxy, dxz, and dyz) containing three orbitals and a higher energy set (dz² and dx²-y²) with two orbitals. 3. The d-electrons from Ni²⁺ ([Ar] 3d⁸) will fill up the orbitals according to Hund's rule (first fill the lower energy orbitals, then the higher energy orbitals with a maximum of two unpaired electrons). In this scenario, the Ni²⁺ ion has 8 d-electrons: 6 will fill the lower energy orbitals, and 2 will fill the higher energy orbitals, leaving 2 unpaired electrons. This aligns with the fact that NiCl₄²⁻ has 2 unpaired electrons.

Construct the Crystal Field Splitting Diagram of Ni(CN)₄²⁻

Since CN⁻ is a strong field ligand, the crystal field splitting diagram for a tetrahedral complex would look like this: 1. There would be no splitting in the 4s and 4p orbitals. 2. The d-orbitals will be split into a lower energy set (dxy, dxz, and dyz) containing three orbitals and a higher energy set (dz² and dx²-y²) with two orbitals. 3. The d-electrons from Ni²⁺ ([Ar] 3d⁸) will fill the orbitals according to the Aufbau principle (starting from the lowest available energy orbitals). In this scenario, the Ni²⁺ ion has 8 d-electrons: all 8 electrons will fill the lower energy orbitals, leaving no unpaired electrons. This aligns with the fact that Ni(CN)₄²⁻ is diamagnetic.

Propose Structures

Now that we know how the d-electrons are filled in each complex ion, we can propose structures for both. NiCl₄²⁻: - Central atom: Ni²⁺ - Ligands: 4 Cl⁻ ions - Geometry: Tetrahedral - Bonding: Ni²⁺ will form covalent bonds with the Cl⁻ ions through the filled d-orbitals. Ni(CN)₄²⁻: - Central atom: Ni²⁺ - Ligands: 4 CN⁻ ions - Geometry: Tetrahedral - Bonding: Ni²⁺ will form covalent bonds with the CN⁻ ions through the filled d-orbitals. Thus, the structures of NiCl₄²⁻ and Ni(CN)₄²⁻ are both tetrahedral, with the difference being the ligands and how the d-electrons are filled in each case. NiCl₄²⁻ has two unpaired electrons, while Ni(CN)₄²⁻ is diamagnetic with no unpaired electrons.

Key concepts

Ligand Field Strength

Ligand field strength is a crucial concept in understanding how different ligands influence the electronic arrangements in complex ions. Ligands can be classified as either weak-field or strong-field based on their ability to split the d-orbitals of the central metal ion.

**Weak-field Ligands:**

• Examples include halides like Cl⁻.
• They create a small energy difference between the d-orbitals.
• Because of this smaller energy gap, the pairing energy of electrons remains relatively high.

**Strong-field Ligands:**

• Examples include CN⁻ and CO.
• They induce a large separation between the higher and lower sets of d-orbitals.
• This larger splitting overcomes the electron pairing energy.

In the case of NiCl₄²⁻ and Ni(CN)₄²⁻ complexes, Cl⁻ acts as a weak ligand, resulting in unpaired electrons, while CN⁻, as a strong ligand, leads to all electrons being paired.

Complex Ions

Complex ions consist of a central metal ion bonded to surrounding molecules or ions called ligands. The formation of complex ions involves the overlap of atomic orbitals from both the metal and ligands, creating covalent bonds.

In a complex such as NiCl₄²⁻:

• The central ion is Ni²⁺, which coordinates with four chloride ions.
• This results in a tetrahedral shape due to the spatial arrangement of ligands.

For Ni(CN)₄²⁻:

• Nickel again acts as the central ion, surrounded by four cyanide ions.
• This also forms a tetrahedral configuration.

These complexes illustrate how different ligands affect geometry and electronic distribution while highlighting critical aspects of electron arrangement in complex ions.

Electron Configuration

Electron configuration in transition metal complexes is significantly impacted by ligand field strength. Understanding electron configuration helps predict properties such as color and magnetism.

For Ni²⁺ (28 electrons as Ni, but 26 as Ni²⁺):

• Initial configuration as Ni is [Ar] 3d⁸4s².
• As Ni²⁺, it loses 2 electrons resulting in [Ar] 3d⁸.

In NiCl₄²⁻ complex:

• With Cl⁻ as a weak ligand, the d-orbitals fill giving rise to unpaired electrons.

In Ni(CN)₄²⁻ complex:

• With CN⁻ as a strong ligand, electrons pair fully in lower energy d-orbitals.

This variation in electron distribution is pivotal in determining magnetic properties of complexes.

Magnetic Properties

Magnetic properties in complexes are determined by the spin states of electron configurations. The number of unpaired electrons plays a key role in defining whether a complex is paramagnetic or diamagnetic.

**Paramagnetic Complexes:**

• Contain one or more unpaired electrons.
• Show attraction to magnetic fields.
• Example: NiCl₄²⁻, due to its two unpaired electrons.

**Diamagnetic Complexes:**

• All electrons are paired.
• Show a weak repulsion from magnetic fields.
• Example: Ni(CN)₄²⁻, as no unpaired electrons are present.

The ligand field strength, as seen in Ni complexes, profoundly influences these magnetic properties by dictating the electron arrangement within the d-orbitals.