Cobalt(III) trifluoroacetylacetonate, Co1tfac23, is a sixcoordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom: (d) Draw a crystal field energy-level diagram for Co1tfac23, and predict its magnetic properties. (In this complex, tfac is a strong-field ligand.)
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Identify the oxidation state of the cobalt ion in the complex. Since the complex is Cobalt(III), the oxidation state of Co is +3.
Determine the electron configuration of the Co(III) ion. Cobalt in its elemental form is [Ar] 3d^7 4s^2. For Co(III), remove three electrons, resulting in [Ar] 3d^6.
Recognize that the complex is octahedral and tfac is a strong-field ligand, which means it will cause a large splitting in the d-orbitals (Δ_oct).
Draw the crystal field splitting diagram for an octahedral complex. The d-orbitals split into two sets: the lower energy t2g (d_xy, d_xz, d_yz) and the higher energy e_g (d_z^2, d_x^2-y^2) orbitals.
Since tfac is a strong-field ligand, the electrons will pair up in the lower energy t2g orbitals before occupying the higher energy e_g orbitals. With 6 electrons, all will pair in the t2g orbitals, resulting in a low-spin configuration, which is diamagnetic (no unpaired electrons).
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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 its electronic structure and energy levels. In octahedral complexes, the d-orbitals split into two energy levels: the lower-energy t2g and the higher-energy eg orbitals. The extent of this splitting depends on the nature of the ligands, with strong-field ligands causing a larger splitting, which influences the complex's magnetic properties.
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, like tfac in this case, cause significant splitting of the d-orbitals, leading to low-spin configurations where electrons pair up in the lower energy orbitals. This property is crucial for predicting the magnetic behavior of the complex, as low-spin complexes tend to be diamagnetic.
Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.
Magnetic Properties of Complexes
The magnetic properties of coordination complexes are determined by the presence of unpaired electrons in their d-orbitals. A complex with unpaired electrons is paramagnetic, while one with all paired electrons is diamagnetic. In the case of Co1tfac23, the strong-field tfac ligands lead to a low-spin state, resulting in no unpaired electrons and thus a diamagnetic property, which can be predicted from the crystal field energy-level diagram.
McMurry 8th Edition
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