Textbook Question
Explain why [CoCl4]2- (blue) and [Co(H2O)6]2+ (pink) have different colors. Which complex has its absorption bands at longer wavelengths?
Ch.21 - Transition Elements and Coordination Chemistry
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McMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.122
McMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.122Chapter 21, Problem 21.122
Predict the crystal field energy-level diagram for a square pyramidal ML5 complex that has two ligands along the axes but only one ligand along the z axis. Your diagram should be intermediate between those for an octahedral ML6 complex and a square planar ML4 complex.
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Identify the geometry of the complex: The complex in question is a square pyramidal ML5 complex. This geometry can be visualized as an octahedral ML6 complex missing one ligand.
Understand the impact of missing ligand: Removing one ligand from an octahedral complex (to form a square pyramidal structure) reduces the symmetry and alters the distribution of the d-orbitals in terms of energy.
Compare with related geometries: The square pyramidal geometry is intermediate between the octahedral (ML6) and square planar (ML4) geometries. In an octahedral field, the d-orbitals split into two groups: t2g (lower energy) and eg (higher energy). In a square planar field, the d-orbitals split differently due to the higher symmetry and planar arrangement.
Predict the splitting pattern: In a square pyramidal complex, expect a splitting pattern that is somewhat a hybrid of the octahedral and square planar patterns. The absence of one axial ligand (compared to the octahedral) will cause the dz2 orbital to have a different energy compared to the dx2-y2 orbital, which is more affected in a square planar field.
Sketch the energy-level diagram: Start with the octahedral diagram as a base. Then, adjust the energies of the d-orbitals considering the reduced symmetry and the influence of having only one ligand along the z-axis. The dz2 orbital will likely be lower in energy compared to its position in a purely octahedral field, and the splitting of the other d-orbitals will need to be adjusted to reflect the intermediate nature of the square pyramidal geometry.
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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 the d-orbitals. In this theory, ligands create an electric field that splits the degenerate d-orbitals into different energy levels based on their spatial orientation relative to the ligands. Understanding CFT is crucial for predicting the electronic structure and color of coordination complexes.
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The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.
Ligand Field Strength
Ligand field strength refers to the ability of a ligand to influence the energy levels of the d-orbitals in a metal complex. Strong field ligands cause a larger splitting of the d-orbitals, while weak field ligands result in smaller splitting. The arrangement of ligands in a square pyramidal geometry, with two ligands along the axes and one along the z-axis, leads to a unique energy-level diagram that reflects intermediate characteristics between octahedral and square planar complexes.
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02:40Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.
Geometry of Coordination Complexes
The geometry of coordination complexes, such as square pyramidal, octahedral, and square planar, determines the spatial arrangement of ligands around the central metal ion. Each geometry has distinct d-orbital splitting patterns, which influence the electronic transitions and properties of the complex. Understanding these geometries is essential for predicting the crystal field energy-level diagram and the resulting chemical behavior of the complex.
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Related Practice
Textbook Question
Look at the colors of the isomeric complexes in Figure 21.12, and predict which is the stronger field ligand, nitro (-NO2) of nitrito (-ONO). Explain.
Textbook Question
Predict the crystal field energy-level diagram for a linear ML2 complex that has two ligands along the :
Textbook Question
Give a valence bond description of the bonding in each of the following complexes. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.
(b) [NiBr4]2- (tetrahedral)
Textbook Question
For each of the following complexes, describe the bonding using valence bond theory. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.
(a) [AuCl4]2 (square planar)
Textbook Question
For each of the following complexes, describe the bonding using valence bond theory. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.
(b) [Ag(NH3)2]+