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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.140d
Chapter 21, Problem 21.140d

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.)

Verified step by step guidance
1
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.
Recommended video:
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01:18
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.
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02:40
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.
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For octahedral complexes, Weak-Field Ligands create High-spin complexes and Strong-Field Ligands create Low-spin complexes.
Related Practice
Textbook Question

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons.

(c) [Co(NCS)4]2- (tetrahedral)

Textbook Question

Nickel(II) complexes with the formula NiX2L2, where X is Cl- or N-bonded NCS- and L is the monodentate triphenylphosphine ligand P(C6H5)3, can be square planar or tetrahedral.

(c) Draw possible structures for each of the NiX2L2 complexes, and tell which ones have a dipole moment.

Textbook Question

Spinach contains a lot of iron but is not a good source of dietary iron because nearly all the iron is tied up in the oxalate complex [Fe(C2O4)3]3-.

(d) Draw the structure of [Fe(C2O4)3]3-. Is the complex chiral or achiral?

Textbook Question

In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)1Cl2]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

(d) Is the reaction product chiral or achiral? Explain.

Textbook Question

In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)2Cl1]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

e. Draw a crystal field energy-level diagram for trans-[Co(en)2Cl2]+ that takes account of the fact that Cl– is a weaker-field ligand than ethylenediamine.

Textbook Question

The complete reaction of 2.60 g of chromium metal with 50.00 mL of 1.200 M H2SO4 in the absence of air gave a blue solution and a colorless gas that was collected at 25°C and a pressure of 735 mm Hg. (e) When an excess of KCN is added to the solution, the color changes, and the paramagnetism of the solution

decreases. Explain.