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Ch.21 - Transition Elements and Coordination Chemistry
Chapter 21, Problem 97

Tris(2-aminoethyl)amine, abbreviated tren, is the tetradentate ligand N(CH2CH2NH2)3. Using to represent each of the three NCH2CH2NH2 segments of the ligand, sketch all possible isomers of the octahedral complex [Co(tren)BrCl]+.

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Identify the coordination number and geometry of the complex. The complex [Co(tren)BrCl]+ has a coordination number of 6, forming an octahedral geometry.
Recognize that the ligand tren is tetradentate, meaning it occupies four coordination sites on the cobalt ion, leaving two sites for the halide ions (Br and Cl).
Consider the possible arrangements of the Br and Cl ligands in the octahedral complex. Since tren occupies four positions, Br and Cl can occupy the remaining two positions.
Determine the possible isomers. In an octahedral complex, the two halide ions can be either adjacent to each other (cis) or opposite each other (trans).
Sketch the isomers: one with Br and Cl in the cis position and another with Br and Cl in the trans position, keeping the tren ligand in a fixed position.

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Key Concepts

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

Tetradentate Ligands

Tetradentate ligands are molecules that can form four bonds with a central metal ion. In the case of tren, it has three amine groups that can coordinate to a metal, allowing it to effectively bind and stabilize the metal ion in a complex. Understanding how these ligands interact with metal ions is crucial for predicting the geometry and properties of the resulting complexes.
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Octahedral Geometry

Octahedral geometry is a common arrangement of six ligands around a central metal ion, forming an octahedron. This geometry is typical for transition metal complexes, where the ligands occupy the vertices of the octahedron. Recognizing the spatial arrangement of ligands in octahedral complexes is essential for visualizing isomerism and understanding the complex's properties.
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Isomerism in Coordination Compounds

Isomerism in coordination compounds refers to the existence of different compounds that have the same formula but different arrangements of atoms or ligands. In octahedral complexes, isomers can arise from variations in ligand positioning, such as facial (fac) and meridional (mer) isomers. Identifying and sketching these isomers requires a clear understanding of how ligands can be arranged around the central metal ion.
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Related Practice
Textbook Question

Write the formula for each of the following compounds.

(c) Hexacarbonylmolybdenum(0)

(d) Diamminebis(ethylenediamine)chromium(III) chloride

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Textbook Question

Which of the following complexes are chiral?

(a) Pt(en)Cl2

(b) cis-[Co(NH3)4Br2]+

(c) cis-[Cr(en)2(H2O)2]3+

(d) [Cr(C2O4)3]3-

Open Question
How does plane-polarized light differ from ordinary light? Draw the structure of a chromium complex that rotates the plane of plane-polarized light.
Textbook Question

Consider the octahedral complex [Co(en)(dien)Cl]2+, where dien = H2NCH2CH2NHCH2CH2NH2, which can be abbreivated

(a) The dien (diethylenetriamine) ligand is a tridentate ligand. Explain what is meant by 'tridentate' and why dien can act as a tridentate ligand.

(b) Draw all possible stereoisomers of [Co(en)(dien)Cl]2+ (dien is a flexible ligand). Which stereoisomers are chiral, and which are achiral?

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Textbook Question

The reaction of the octahedral complex Co(NH3)3(NO2)3 with HCl yields a complex [Co(NH3)3(H2O)Cl2]+ in which the two chloride ligands are trans to one another.

(a) Draw the two possible stereoisomers of the starting material [Co(NH3)3(NO2)3]. (All three NO2- ligands are bonded to Co through the N atom.)

(b) Assuming that the NH3 groups remain in place, which of the two starting isomers could give rise to the observed product?

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Open Question
Use a sketch to explain why the dxy and dx2-y2 orbitals have different energies in an octahedral complex. Which of the two orbitals has higher energy?