Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following complex species does not involve inner orbital hybridization? (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) (b) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) (c) \(\left[\mathrm{CoF}_{6}\right]^{3-}\) (d) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3}\)

Step-by-Step Solution

Answer

\\([\mathrm{CoF}_{6}]^{3-}\\) does not involve inner orbital hybridization.

Step 1: Understand Inner and Outer Orbital Hybridization

Inner orbital or low-spin complexes involve the pairing of electrons in the t2g orbitals, usually resulting in d2sp3 hybridization. Meanwhile, outer orbital or high-spin complexes use outer 3d orbitals, resulting in sp3d2 hybridization. Identifying the presence of strong or weak ligands can assist in determining the type.

Step 2: Analyze \\([\mathrm{Co}(\mathrm{NH}_{3})_{6}]^{3+}\\)

Cobalt in \([\mathrm{Co}(\mathrm{NH}_{3})_{6}]^{3+}\) is in the +3 oxidation state, with an electronic configuration of \([\mathrm{Ar}] 3d^6\). Ammonia (NH3) is a strong field ligand, causing pairing of electrons, leading to inner d2sp3 hybridization.

Step 3: Analyze \\([\mathrm{Cr}(\mathrm{NH}_{3})_{6}]^{3+}\\)

Chromium in \([\mathrm{Cr}(\mathrm{NH}_{3})_{6}]^{3+}\) is also with +3 oxidation, having an electronic configuration of \([\mathrm{Ar}] 3d^3\). NH3 being a strong field ligand pairs electrons for inner orbital hybridization d2sp3.

Step 4: Analyze \\([\mathrm{CoF}_{6}]^{3-}\\)

Cobalt in \([\mathrm{CoF}_{6}]^{3-}\) with +3 oxidation has \([\mathrm{Ar}] 3d^6\) configuration. Fluoride (F) is a weak field ligand causing no electron pairing, leading to outer orbital sp3d2 hybridization.

Step 5: Analyze \\([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\\)

Iron in \([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\), with +3 oxidation, has an electronic configuration of \([\mathrm{Ar}] 3d^5\). Cyanide (CN-) is a strong field ligand, resulting in electron pairing and inner d2sp3 hybridization.

Step 6: Identify the Exception

Among the given complexes, \([\mathrm{CoF}_{6}]^{3-}\) does not involve inner orbital hybridization but rather outer orbital hybridization because of the weak field ligand fluoride.

Key Concepts

Inner Orbital Hybridization

Inner orbital hybridization refers to a specific mixing of atomic orbitals that often occurs in transition metal complexes when strong field ligands are present. In this type of hybridization, electrons are paired up in the metal's inner 3d orbitals (particularly the t2g set) before going out to hybridize. This leads to the formation of d2sp3 hybrid orbitals. Typically, this results in low-spin configurations, as electrons are tightly paired.

• Occurs with strong field ligands, like cyanide (CN^-) or ammonia (NH₃).
• Forms d2sp3 hybrid orbitals.
• Resulting complexes are usually low-spin.

Understanding this concept is vital because it indicates the strength of the interaction between the metal center and the ligands, which influences several properties, including magnetism and color of the complex.

Outer Orbital Hybridization

Outer orbital hybridization is seen in transition metal complexes when weak field ligands are around. In contrast to inner orbital hybridization, this type uses the metal's outer or higher energy d orbitals (like 4d), leading to sp3d2 hybridization. Because the electrons do not pair in inner t2g orbitals, they remain unpaired, resulting in high-spin configurations.

• Facilitated by weak field ligands, such as fluoride (F^-).
• Leads to sp3d2 hybrid orbitals.
• Complexes usually end up in high-spin states.

This concept contrasts sharply against inner orbital hybridization and emphasizes how the choice of ligand can drastically alter the behavior of a coordination compound.

Field Strength of Ligands

Ligands in coordination chemistry can be classified by their ability to influence the distribution of electrons in the metal atom or ion, a property known as "field strength." The field strength of a ligand determines whether a complex will exhibit inner orbital or outer orbital hybridization.

• Strong field ligands: They cause pairing of electrons (e.g., CN^-, NH₃).
• Weak field ligands: They do not cause electron pairing (e.g., F^-).

Understanding whether a ligand is strong or weak is key to predicting the type of hybridization and consequently the geometry, magnetism, and other physical properties of the complex. Field strength is fundamental to the broad category of concepts known as "ligand field theory."

Electronic Configuration

The electronic configuration of the central metal atom/ion in a complex provides a blueprint for determining its chemical behavior and properties. It indicates how electrons are arranged around the nucleus, commonly represented using notations based on the Aufbau principle. For transition metals, the 3d and 4s orbitals are particularly important.

• Influences the hybridization type within the complex.
• Determines magnetic and optical properties.

Taking an example, Co³⁺ would typically have the configuration ([Ar] 3d⁶:
) depending on the ligands around it, the electrons may undergo pairing (inner orbital formation) or remain unpaired (outer orbital formation). Identifying configurations helps predict whether the complex is paramagnetic or diamagnetic, among other characteristics.

High-spin and Low-spin Complexes

In coordination chemistry, the concepts of high-spin and low-spin complexes arise from the interactions between the metal ion and the ligands. These interactions affect the distribution of electrons among the d-orbitals.

• High-spin complexes: Occur with weak field ligands leading to minimal electron pairing and maximum unpaired electrons.
• Low-spin complexes: Result from strong field ligands, causing electron pairing and fewer unpaired electrons.

The spin state of a complex has significant impacts on its properties:

• Magnetism: High-spin complexes are typically paramagnetic, while low-spin are diamagnetic or less paramagnetic.
• Color: The difference in d-electron arrangements affects light absorption, influencing the color of the complex.

Mastering the distinction between high-spin and low-spin is essential for understanding the behavior of many complex compounds in coordination chemistry.