Ch.21 - Transition Elements and Coordination Chemistry

Problem 21.140b

Chapter 21, Problem 21.140b

Cobalt(III) trifluoroacetylacetonate, Co(tfac)₃, is a sixcoordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom:
(b) Diastereoisomers A and B have dipole moments of 6.5 D and 3.8 D, respectively. Which of your diastereoisomers is A and which is B?

Verified step by step guidance

Identify the structure of the octahedral complex Co(tfac)_3, where tfac is a bidentate ligand, meaning each tfac ligand forms two bonds with the central Co atom.

Understand that diastereoisomers are stereoisomers that are not mirror images of each other, and in octahedral complexes, they can arise from different spatial arrangements of the ligands.

Consider the possible arrangements of the three tfac ligands around the cobalt center. In an octahedral complex, these can lead to different spatial configurations, such as facial (fac) and meridional (mer) isomers.

Recall that the dipole moment is a measure of the separation of positive and negative charges in a molecule. A higher dipole moment indicates a more polar molecule, which can be influenced by the spatial arrangement of the ligands.

Assign the diastereoisomers based on their dipole moments: the isomer with the higher dipole moment (6.5 D) is likely to have a less symmetrical arrangement of ligands, while the isomer with the lower dipole moment (3.8 D) is likely to have a more symmetrical arrangement.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Diastereoisomers

Diastereoisomers are a type of stereoisomer that are not mirror images of each other. They occur in compounds with multiple chiral centers, where the configuration at one or more centers differs while others remain the same. This results in distinct physical properties, such as different dipole moments, which can be used to differentiate between them.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating its polarity. It is a vector quantity, with both magnitude and direction, and is expressed in Debye units (D). A higher dipole moment generally suggests a more polar molecule, which can influence its interactions and stability.

Octahedral Coordination

Octahedral coordination refers to a molecular geometry where a central atom is surrounded by six ligands at the corners of an octahedron. This arrangement is common in transition metal complexes, such as cobalt(III) trifluoroacetylacetonate, and affects the electronic and steric properties of the complex, influencing its reactivity and interactions with other molecules.

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Related Practice

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

Cobalt(III) trifluoroacetylacetonate, Co(tfac)₃, is a sixc oordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom:

(a) Draw all possible diastereoisomers and enantiomers of Co(tfac)₃.

Textbook Question

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.)

Textbook Question

Based on effective nuclear charge (Z_eff), which ion is the strongest oxidizing agent?

(a) Cu²⁺

(b) Ni²⁺

(d) Mn²⁺

Textbook Question

Look at the location in the periodic table of elements A, B, C, and D. What is the electron configuration of the transition metal in each of the following ions?

(a) A²⁺

(b) B⁺