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PAPER No.7:Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) MODULE No. : 20 (Isomerism part-II)

Subject Chemistry

Paper No and Title Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes)

Module No and Title 20: Isomerism part II

Module Tag Geometrical isomerism

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TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 2.1 Linkage Isomerism 2.2 Ligand Isomerism 3. Stereoisomerism 3.1 Geometrical Isomerism 3.1.1 cis–trans isomerism

3.1.2Identification of cis-& trans- isomers 3.1.3 facial–meridional isomerism

4. Solved problems 5. Summary

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Prerequisites

Before going into the details of this module we should be aware of definition and types of isomerism. That is

Isomerism in Coordination Complexes

The compounds having identical empirical formula but different physical and chemical properties are known as isomers. This phenomenon is known as isomerism. Two principal types of isomerism are known among coordination compounds. Each of one can be further sub-divided.

1. Constitutional / Structural Isomerism

(a) Coordination isomerism, (b) Polymerization isomerism, (c) Ionization isomerism, (d) Hydrate isomerism, (e) Linkage isomerism and (f) Ligand isomerism

2. Stereoisomerism

(a) Geometrical isomerism and (b) Optical isomerism

We have already studied (a) Coordination isomerism, (b) Polymerization isomerism, (c) Ionization isomerism, (d) Hydrate isomerism in detail. In this module, we will start our discussion from linkage isomerism.

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1. Learning Outcomes

After studying this module, you shall be able to

• Know about linkage and ligand isomerism in coordination complexes, as a part of constitutional isomerism

• Learn about geometrical isomerism in octahedral as well as tetrahedral complexes • Identify cis-trans isomers and fac-mer isomers • Evaluate the number of geometrical isomers possible for a given empirical formula

2. Introduction

Constitutional / Structural isomers are molecules that have the same number and type of atoms, but they are attached in a different order. These isomers have very different physical and chemical properties. There are several forms of structural isomerism. Linkage isomerism and ligand isomerism are described below.

2.1Linkage isomerism- Linkage isomers differ in the atom of a ligand bonded to the metal in the complex. This type of isomerism occurs with ambidentate ligands.These ligands are capable of coordinating to two metal centres simultaneously in more than one way. The best known cases involve the ambidentate ligands like SCN- / NCS- and NO2

- / ONO-. For example, in the complex [Co(NH3)5NO2]2+, ligand NO2

- forms nitro bonds via nitrogen(NO2-), and nitrito via an oxygen

(NO2-) atom. The nitro compound is yellow in color, however, the nitrito compound is red

(Figure 1).

Figure 1

The nitro isomer (yellow) is stable enough when it is stored away from direct sunlight, however, on exposure to the sunlight (UV radiation) leads to the isomerisation to the nitrito isomer (red). Upon standing, the nitrito isomer converts completely back to the nitro isomer. The mechanism for this interconversion is believed to be intramolecular.

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[Co(NH3)5NO2]2+ [Co(NH3)5ONO]2+

Important factors that decide the formation of the particular linkage isomers under certain conditions are:

(a) The nature of metal ion

(b) The electronic and steric effects of the ambidentate ligand or other ligands in the complex

(c) The solvent system used in isomer synthesis

(d) The solvent system used in the recrystallization of the isomer.

The first factor is reflected in our next example where Pd(II) form S-bonded complex [Pd(SCN)4]2- with SCN-, however, the analogous complex formed by Cd(II) has both N- andS-bonded SCN-. On the other hand, Co(III) forms N-bonded complex [Co(NH3)5 NCS]2+ with SCN-. The scheme is shown in Figure 2.

Figure 2

Another example (Figure 3) shows the effect of steric and electronic factors of other ligands attached to the metal center on the coordination behavior of ambidentate ligand. For example, in complex [Co(NH3)5NCS]2+, when NH3 ligands are replaced with CN-, the N-bonded complex change to S- bonded complex [Co(CN)5 SCN]3-.

Figure 3

hν

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2.2Ligand Isomerism- As the name implies, ligand isomerism arises from the presence of ligandswhich itself are isomers to each other. For example, diaminopropane can have the amine groups at two different positions giving rise to two isomers: 1,3-diaminopropane (H2NCH2CH2CH2NH2) and 1,2-diaminopropane (H2NCH2-CH(CH3)NH2). These two ligands coordinate differently with the metal ions. 1,3-diaminopropane and 1, 2 -diaminopropane form complexes, [Pt(H2NCH2CH2CH2NH2)2]2+ (Figure 4) and [Pt(H2NCH2CH (CH3)NH2)2]2+(Figure 4), respectively. These complexes are ligand isomers. Another example is [CoCl(en)2(NH2C6H4Me)]Cl2, in this complex coordinated toluidine may be of the o-, m- or p-form.

Figure 4

3. Stereoisomerism

Stereoisomers are molecules that have the same number and type of atoms, and that are attached in the same order, but the atoms or groups of atoms point in a different spatial direction. There are two types of stereoisomerism:

a) Geometrical isomerism

b) Optical isomerism

3.1 Geometrical Isomerism

Geometrical isomers differ in the geometric arrangements of the ligands about the central metal ion. They have different chemical and physical properties. They also have different colors, melting points, polarity, solubility, reactivity, etc. Geometrical isomers exist as distinct compounds because there is no low energy path for their interconversion. Geometrical isomers are possible for both square planar and octahedral complexes, but not for tetrahedral geometries. There are two types of geometrical isomerism possible for coordination compounds.

3.1.1 Cis–trans isomerism

In a complex, when two monodentate ligands are mutually adjacent they are said to be cis-, when opposite to each other, trans. Cis–trans isomerism occurs in square planar complexes and octahedral complexes of the type MA4B2 or typeMA2B4, but this isomerism does not occur in tetrahedral complexes.

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For square planar complexes, MA2B2, there are two possible isomers cis- where both A ligands are on the same side as the ligands B (Figure 5). However for trans, the A ligands can be across from one another, in which case the B ligands must also be trans (Figure 5). Even though it is possible to draw the cis-isomer in four different ways and the trans isomer in two different ways, all members of each set are chemically equivalent.

Figure 5

Most common examples of cis and trans isomers of an MA2B2 type square planar complex are cis-Pt(NH3)2Cl2, also known as cisplatin (Figure 6), and trans-[Pt(NH3)2Cl2] (Figure 7). For the cis-isomer both Cl- groups are on the same side and the both NH3 groups are also on the same side (Figure 6). However for trans, both Cl- groups are across from each other whereas two NH3 groups are also trans to each other (Figure 7). Cisplatin is an anticancer drug, however, trans isomer is actually toxic rather than therapeutic. Such a stark difference illustrates the different properties exhibited by the two geometrical isomers.

Figure 6

Figure 7

For octahedral complex MA4B2, two ligands A are different from the other four ligands B, giving two possible isomers. The two B ligands can be cis or trans to each other (Figure 8).

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Figure 8

cis- and trans-[Co(NH3)4Cl2]+ complexes are examples of this type of isomerism. The cis-[Co(NH3)4Cl2]+ ion (Figure 9) is red in color, whereas, trans-[Co(NH3)4Cl2]+(Figure 10) is green.

Figure 9

Figure 10

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cis-trans isomerism is also possible with bidentate ligands; if bidentate ligands are symmetrical then the resultant square planar complex will not show geometrical isomerism. For example -

[M(AA)2]n+(AA= symmetrical bidentate ligand, no geometrical isomerism

[M(AB)2]n+(AB= unsymmetrical bidentate ligand, geometrical isomerism

Square planar complexes that contain symmetrical bidentate ligands, such as [Pt(en)2]2+, have only one possible structure (Figure 11) in which curved lines linking the two N atoms indicate the ethylenediamine ligands. However, square planar complexes containing unsymmetrical bidentate ligands, such as [Pt(gly)2]2+, give two geometrical isomers (Figure 12). In these examples, “en” refers to ethylene diamine (H2NCH2CH2NH2), while “gly” to glycine (HOOCCH2NH2),

Figure 11

Figure 12

The octahedral complexes of the type [M(AA)3]n+ having symmetrical bidentate ligands do not show geometrical isomerism, however, complexes, [M(AA)2B2]n+and [M(AA)B2C2]n+give two geometric isomers each. For example [Cr(en)2(CN)2]+(Figure 13) and [Cr(en)(NH3)2Cl2]+(Figure 14).

Figure 13

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Figure 14

On the other hand, octahedral complexes of the type [M(AB)3]n+ having unsymmetrical bidentate ligands also show geometrical isomerism. e.g. [Co(gly)3](Figure 15) where“gly” refers to glycine (HOOCCH2NH2).

Figure 15

3.1.2 Identification of cis- & trans- isomers Identification of cis- & trans- isomers is possible by using following methods: 1. Chemical reaction- There are two chemical methods to identify such isomers. First is Grinsberg’s method and second one is Kunakov’s method. Grinsberg method is based on reaction of the isomers with bidentate ligand. cis isomers generally react with bidentate ligand immediately, however, trans will not react or reaction is slow. Kunakov’s method is based on trans effect. 2. Dipole Moment- cis-isomers usually have higher value of dipole moment relative to trans- isomers. Thus, on the basis of their dipole moment, these isomers can be identified. 3. Infrared (IR) spectroscopy-The compounds having permanent dipole moment are IR active, so they can be identified on the basis of IR spectrum. 4. Magnetic Moment- cis-isomers will have higher value of magnetic moment as compared to the trans- isomers. Thus on the basis of their magnetic moment, isomers can be identified. 5. X-ray crystallography- Single crystal X-ray diffraction is the most powerful tool for detailed structural analysis of crystalline coordination compounds. This method can be used for the identification of cis-/trans- isomers.

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3.1.3Facial–meridional isomerism

When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac. In a fac isomer, any two identical ligands are adjacent or cis to each other. If such three ligands and the metal ion are in one plane, the isomer is said to be meridional, or mer. A mer isomer can be considered as a combination of a trans and a cis, since it contains both trans and cis pairs of identical ligands. fac–mer isomerism occurs in MA3B3 type octahedral compounds. For an octahedral complex of the type, MA3B3, two isomers are possible (Figure 16). In one, the three same ligands (B), occupy opposite triangular faces of the octahedron; this is called the fac- isomer (for facial). However, in another isomer, the three same ligands (B) would be at the meridian if the complex was viewed as a sphere; this is called the mer- isomer (for meridional). In other words,

fac- three identical ligands are on one triangular face and

mer- three identical ligands are in a plane bisecting the molecule.

Figure 16

fac- and mer-[Co(NH3)3Cl3] are examples of this type of isomerism. In the first isomer, fac-[Co(NH3)3Cl3] (Figure 17),three Cl- ligands are on one triangular face. In another isomer, mer-[Co(NH3)3Cl3] (Figure 18),three Cl- ligands are in a plane which is bisecting the molecule.

Figure 17

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Figure 18

4.Solved problems

Question 1: Draw all the possible geometrical isomers for the complex [Fe(NH3)2(en)BrCl]−, where en is ethylenediamine. Solution: This complex contains one bidentate ligand (ethylenediamine), which can occupy only adjacent (cis) positions, and four monodentate ligands, two of which are identical (NH3). The easiest way to attack the problem is to go through the various combinations of theligands systematically to determine which ligands can be trans. Thus either the NH3 ligands can be trans to one another or the two halide ligands can be trans to one another, giving the two possible geometrical isomers as shown below:

In addition, two additional structures are possible in which one of the halide is trans toNH3 ligand. In the first, the chloride ligand is in the same plane as the ethylenediamine ligand and trans to one of the ethylenediamine nitrogen. Exchanging the chloride and bromide ligands gives the other isomer, in which the bromide ligand is in the same plane as the ethylenediamine ligand and trans- to one of the ethylenediamine nitrogen:

This complex can therefore exist in four different geometrical isomers.

Question 2:Draw all the possible geometrical isomers for the complex [Cr(ox)2(NH3)2]-.

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Answer:

Two geometrical isomers are possible: cis and trans

5.Summary

In this module, we discussed about

• Linkage and ligand isomerism in coordination complexes, as a part of constitutional isomerism

• Geometrical isomerism in coordination complexes • Geometrical isomerism is further divided into two types: cis-trans isomerism and fac-mer

isomerism. • Geometrical isomers have different chemical and physical properties • Based on these properties, identification of cis-& trans- isomers by using different

chemical and physical methods

chemistry paper no.7: electronic spectra and …epgp.inflibnet.ac.in/.../et/5485_et_et.pdfisomerism...

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