Textbook Question
Which of the following complexes can exist as enantiomers? Draw their structures.
(a) [Cr(en)3]3+
(b) cis-[Co(NH3)Cl]2+
(c) trans-[Co(en)2(NH3)Cl]2+
(d) [Pt(NH3)3Cl3]+
Ch.21 - Transition Elements and Coordination Chemistry
Chapter 21, Problem 21.91
Draw the three possible diastereoisomers of the triethylenetetramine complex [Co(trend)Cl2]+. Abbreviate the flexible tetradentate trien ligand H2NCH2CH2NHCH2CH2NHCH2CH2NH2 as . Which of the isomers can exist as a pair of enantiomers?
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Identify the coordination geometry of the complex. Since cobalt typically forms octahedral complexes, assume an octahedral geometry for [Co(trien)Cl_2]^+.
Consider the coordination of the trien ligand. The trien ligand is a tetradentate ligand, meaning it can form four bonds with the cobalt center, leaving two positions for the chloride ions.
Draw the possible arrangements of the trien ligand and the two chloride ions around the cobalt center. In an octahedral complex, the ligands can be arranged in different spatial configurations, leading to different diastereoisomers.
Identify the three possible diastereoisomers. These will include the facial (fac) and meridional (mer) isomers, where the chloride ions can be adjacent (fac) or opposite (mer) to each other.
Determine which of the diastereoisomers can exist as a pair of enantiomers. Enantiomers are non-superimposable mirror images, so look for a diastereoisomer that lacks a plane of symmetry, which would allow for chirality.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Diastereoisomers
Diastereoisomers are stereoisomers that are not mirror images of each other. They occur in molecules with multiple chiral centers, where at least one but not all chiral centers differ between the isomers. Understanding diastereoisomerism is crucial for analyzing the spatial arrangement of ligands in coordination complexes, such as the triethylenetetramine complex in the question.
Chirality and Enantiomers
Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image, leading to the formation of enantiomers. Enantiomers are pairs of chiral molecules that are mirror images of each other and have identical physical properties except for their interaction with polarized light. Identifying which diastereoisomers can exist as enantiomers is essential for understanding the optical activity of the complex.
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Coordination Chemistry
Coordination chemistry studies the structures and properties of coordination compounds, which consist of a central metal atom bonded to surrounding ligands. The geometry and arrangement of these ligands around the metal can lead to different isomeric forms, including diastereoisomers. In the context of the question, recognizing how the tetradentate ligand coordinates with cobalt is vital for drawing the correct isomers.
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