Textbook Question
A classmate says, “A weak-field ligand usually means the complex is high spin.” Is your classmate correct? Explain.
Ch.23 - Transition Metals and Coordination Chemistry
Chapter 23, Problem 63
Complete the exercises below. Draw the crystal-field energy-level diagrams and show the placement of d electrons for each of the following: a. [Cr(H₂O)₆]²⁺ (four unpaired electrons).
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Identify the oxidation state of the central metal ion, Cr, in the complex [Cr(H₂O)₆]²⁺. Since water is a neutral ligand, the charge on Cr must be +2 to balance the overall charge of the complex.
Determine the electron configuration of the Cr²⁺ ion. Chromium in its elemental form is [Ar] 3d⁵ 4s¹. When it loses two electrons to form Cr²⁺, the electron configuration becomes [Ar] 3d⁴.
Recognize that [Cr(H₂O)₆]²⁺ is an octahedral complex. In an octahedral field, the five d orbitals split into two sets: the lower-energy t₂g set (d_xy, d_xz, d_yz) and the higher-energy e_g set (d_z², d_x²-y²).
Place the four d electrons of Cr²⁺ into the crystal-field energy-level diagram for an octahedral complex. Start by filling the lower-energy t₂g orbitals with electrons, following Hund's rule to maximize the number of unpaired electrons.
Verify that the placement of electrons results in four unpaired electrons, as stated in the problem. This confirms the high-spin configuration of the complex, consistent with the presence of weak-field ligands like water.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Crystal Field Theory
Crystal Field Theory (CFT) explains how the arrangement of ligands around a central metal ion affects the energy levels of its d orbitals. In an octahedral complex like [Cr(H₂O)₆]²⁺, the d orbitals split into two energy levels: the lower-energy t₂g and the higher-energy e_g orbitals. This splitting is crucial for understanding the electronic configuration and magnetic properties of the complex.
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01:18
The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.
Electron Configuration
The electron configuration of an atom describes the distribution of electrons among the various atomic orbitals. For transition metals, this involves the d orbitals, which can hold up to ten electrons. In the case of [Cr(H₂O)₆]²⁺, chromium loses two electrons from its 3d orbitals, resulting in a configuration that influences the number of unpaired electrons and the overall magnetic behavior of the complex.
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01:33Electron Configuration Example
Ligand Field Strength
Ligand field strength refers to the ability of ligands to influence the energy levels of the d orbitals in a metal complex. Strong field ligands, like CN⁻, cause a larger splitting of the d orbitals, while weak field ligands, like H₂O, result in smaller splitting. The nature of the ligands affects the number of unpaired electrons and the magnetic properties of the complex, which is essential for drawing the correct energy-level diagram.
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02:40Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.
Related Practice
Textbook Question
For a given metal ion and set of ligands, is the crystal-field splitting energy larger for a tetrahedral or an octahedral geometry?
Textbook Question
Complete the exercises below. Draw the crystal-field energy-level diagrams and show the placement of electrons for each of the following complexes:
a. [VCl6]3–,
b. [FeF6]3– (a high-spin complex),
Textbook Question
Draw the crystal-field energy-level diagrams and show the placement of electrons for each of the following complexes:
d. [NiCl4]2+ (tetrahedral),