CHEMISTRY PAPER No.7: Electronic Spectra and …epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/chemistry/... · Electronic Spectra and Magnetic Properties of ... For this complex,

____________________________________________________________________________________________________

CHEMISTRY

PAPER No.7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) MODULE No. : 19 (Isomerism)

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 19:Isomerism Part I

Module Tag Constitutional / Structural isomerism

____________________________________________________________________________________________________

CHEMISTRY


TABLE OF CONTENTS 1. Learning Outcomes 2. Isomerism in Coordination Complexes 3.Constitutional / Structural Isomerism 3.1 Coordination Isomerism 3.1.1 Separation of Coordination Isomers 3.2 Polymerization Isomerism

3.3 Ionization Isomerism 3.4 Solvate or Hydrate Isomerism 3.5 Coordination Position Isomerism 3.6 Linkage Isomerism 3.7 Ligand Isomerism

4. Solved problems

5. Stereoisomerism

5.1 Geometrical Isomerism 5.2Optical Isomerism

6. Summary

____________________________________________________________________________________________________

CHEMISTRY


1. Learning Outcomes

After studying this module, you shall be able to

• Know about the complexes having same chemical formula but different structural arrangements.

• Learn the types of structural isomerism in detail • Identify the number of isomers possible for the given empirical formula • Evaluate the type of isomerism possible for the given empirical formula

2. Isomerism in Coordination Complexes

The compounds having identical empirical formula but differing in their 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 subdivided.

1. Constitutional and/or Structural Isomerism

a) Coordination isomerism b) Polymerization isomerism c) Ionization isomerism d) Hydrate isomerism e) Coordination position isomerism f) Linkage isomerism g) Ligand isomerism 2. Stereoisomerism a) Geometrical isomerism b) Optical isomerism

3. Constitutional and/or Structural isomerism

Constitutional and/or Structural isomers are the molecules that have the same number and type of atoms, but attached in a different order. These isomers have very different physical and chemical properties. There are numerous forms of structural isomerism, but only seven of the most common ones are described below.

3.1 Coordination isomerism – This type of isomerism is found in special cases where both cation and anion part in the compound are complexes. Now, the metal center of both the cation and anion part can act as coordination center. Isomerism arises from a different distribution of ligands between the two metal ions. Thus, the two isomers differ only in the distribution of ligands between the cation and anion part of the compound.

____________________________________________________________________________________________________

CHEMISTRY


For example a pair of [Co(NH3)6][Cr(CN)6] vs. [Cr(NH3)6][Co(CN)6] form a coordination isomers (Figure 1). Intermediate types between these extremes are also possibleby the exchange of NH3 and CN- ligands between Co(III) of cationic complex and Cr(III) of anionic complex to form two coordination isomers (Figure 1). Two more isomersare possible for the same empirical formula;[Co(NH3)5(CN)][Cr(NH3)(CN)5] and[Co(NH3)4(CN)2][Cr(NH3)2(CN)4]. However, both [Co(NH3)3(CN)3] and [Cr(NH3)3(CN)3] are neutral complexes, so they will act as independent complexes.

Figure 1

Some additional examples of coordination isomerism are as follows: (a) Different metal ions and bidentate ligands:[Co(en)3][Cr(C2O4)3] and [Co(en)2(C2O4)][Cr(en)(C2O4)2] is an example for this type of isomerism. (Figure 2 and Figure 3) (Where en refers to ethylenediamine and C2O4 refers to oxalate ligands) In this case also, one more isomer, [Cr(en)2(C2O4)] [Co(en) (C2O4)2] is possible between these two extremes for the same empirical formula.

Figure 2

Figure 3

(b)Identical metal ions but different ligands: .e.g. [Co(NH3)4(NO2)2][Co(NH3)2(NO2)4] (Figure 4) and [Co(NH3)6][Co(NO2)6] (Figure 5). Another possible isomer is [Co(NH3)5(NO2)][Co(NH3)(NO2)5] for the same empirical formula.

____________________________________________________________________________________________________

CHEMISTRY


Figure 4

Figure 5

(c) Identical metal ions in different oxidation states, but different ligands: example [PtIV(NH3)4Cl2][PtIICl4] (Figure 6) vs. [PtIV(NH3)4][PtIICl6] (Figure 7)

Figure 6

Figure 7

3.1.1 Separation of the coordination isomers: Coordination isomers can be separated by ion exchange method. Using this method, one can separate cations and anions easily. The solution of the complex is first passed through cationic exchange resin and then through anion exchange resin for step-wise separation. 3.2 Polymerization isomerism- It is the special case of coordination isomerism. Polymerization isomerism arises when a series of compounds have the same empirical formula but different molecular masses. It is not a conventional polymerization involving the linking of a single repeating unit. In octahedral complexes with the formula [MXxBb]n , polymerization isomers are

____________________________________________________________________________________________________

CHEMISTRY


often possible. An example of the polymerization isomerism is presented by the series of compounds in which both the cation and anion contain Co3+ and which have the same empirical formula. The value of n is greater than or equal to 1. Several compounds of the empirical formula {Co(NH3)3(NO2)3}n can be isolated as following:

[Co(NH3)3(NO2)3] (n = 1)

[Co(NH3)4(NO2)2]+[Co(NH3)2(NO2)4]− ( n = 2)

[Co(NH3)6]3+ [Co(NO2)6]3− ( n = 2)

[Co(NH3)5(NO2)]2+[Co(NH3)2(NO2)4]2−(n= 3)

[Co(NH3)6]3+[Co(NH3)2(NO2)4]3− (n= 4)

Thus, there are five isomers possible for the same empirical formula.

Another example for polymerization isomerism is compound of empirical formula {Pt(NH3)2Cl2}n that can have following isomers:

[Pt(NH3)2Cl2](n = 1)

[Pt(NH3)4][PtCl4](n = 2)

[Pt(NH3)4][Pt(NH3)Cl3]2(n = 3)

[Pt(NH3)3Cl]2[PtCl4](n = 3)

So there are four isomers possible for the same empirical formula.

3.3 Ionization isomerism – The compounds with same empirical formula and composition but giving different ions in the solution are termed as ionization isomers. This type of isomerism occurs when counter ion of the complex is also a potential ligand. For example -pentaaminebromidocobalt(III)sulfate, [Co(NH3)5Br]SO4 (Figure 8) is red violet and in solution gives a precipitate with barium chloride, confirming presence of the sulfate ion, on the other hand, pentaaminesulfatecobalt(III)bromide [Co(NH3)5SO4]Br (Figure 8) is red and tests negative for sulfate ion in solution, instead gives a yellow precipitate of AgBr with silver nitrate.

____________________________________________________________________________________________________

CHEMISTRY


Figure 8

Consider the following precipitation reaction:

[Co(NH3)5Br]SO4(aq) + BaCl2(aq) → [Co(NH3)5Br]Cl(aq) + BaSO4(s) (white ppt)

[Co(NH3)5(SO4)]Br(aq) + BaCl2(aq) → No precipitation

[Co(NH3)5Br]SO4(aq) + AgNO3(aq) → No precipitation

[Co(NH3)5(SO4)]Br(aq) + AgNO3(aq) → [Co(NH3)5Br]NO3(aq) + AgBr(s) (yellow ppt)

Similarly, [Co(NH3)5Cl]Br and [Co(NH3)5Br]Cl in solution gives a yellow precipitate(AgBr)and white precipitate (AgCl), respectively with silver nitrate.

Consider the following precipitation reaction:

[Co(NH3)5Cl]Br(aq) + AgNO3(aq) → [Co(NH3)5Cl]NO3(aq) + AgBr(s) (yellow ppt)

[Co(NH3)5Br]Cl(aq) + AgNO3(aq) → [Co(NH3)5Br]NO3(aq) + AgCl(s) (white ppt)

Other examples of ionization isomerism are pair of [Pt(NH3)4Cl2]Br2Vvs.[Pt(NH3)4Br2]Cl2 and [Co(en)2(NO2)Cl]SCN, [Co(en)2(NO2)SCN] Cl and [Co(en)2(SCN)Cl]NO2.

3.4 Solvate or hydrate isomerism – These isomers have the same composition but different number of solvent molecules inside as well as outside the coordination sphere. The water molecule inside the coordination sphereare called water of coordination; while those outside the coordination sphere are known as water of crystallization.Water of crystallization escapes from the molecule upon heating below or around 100°C. On the other hand, coordinated water molecules are lost at ∼150°C or even higher temperature.

The best known example of solvate isomerism is chromium chloride "CrCl3.6H2O" (commercially known as Recoura’s Green Chloride) which may contain 4, 5, or 6 coordinated water molecules. For example [Cr(H2O)4Cl2]Cl·2H2O (1) (dark green crystals) is obtained from HCl solution. When complex (1) is dissolved in water it converts into [Cr(H2O)5Cl]Cl2·H2O (1) (blue-green). Complex (2) on heating converts to [Cr(H2O)6]Cl3 (violet). These isomers have very

____________________________________________________________________________________________________

CHEMISTRY


different chemical properties and on reaction with AgNO3 to test for Cl- ions, would give 1, 2, and 3 equivalents of AgCl respectively (Figure 9).

Figure 9

There are some compounds which can show more than one isomerism together in the same compound. For example complex with the empirical formula, Co(H2O)5Br(SO4), can show ionization isomerism as well as hydrate isomerism together. Two possible isomers for this compound are [Co(H2O)5Br]SO4 and [Co(H2O)4 (SO4)]Br.(H2O), as shown in figure 10.

Figure 10

3.5 Coordination position isomerism-Coordination position isomerism is observed in bridged polynuclear complexes. Interchange of ligands takes place between two different metal centers. It is similar to coordination isomerism. A bridged binuclear Co(III) complex is shown in Figure11. For this complex, two isomers can be generated by the exchange of the NH3 and Cl- ligands between two Co(III) centers.

Figure 11

3.6 Linkage 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

H2O

Heat AgNO3 AgNO3

AgNO3

AgCl

2 AgC

l

3 AgC

l

____________________________________________________________________________________________________

CHEMISTRY


of coordinating two metal centers simultaneously in more than one way. The best known cases involve the ambidentate ligands like SCN- / NCS- and NO2

- / ONO-.

3.7 Ligand Isomerism- As the name implies, ligand isomerism arises from the presence of ligands which itself are isomers to each other.

4. Solved problems

Question 1.Using only NH3 and NO2

-, as ligands, give the formula for an octahedral coordination compound containing Co(III) that upon dissolution in water will give (a) two ions , (b)three ions (c) four ions. Solution: Co(III)form octahedral complex with NH3 and NO2

- ligands. We can have several combination of the two ligands around the central metal ion: (I) Six NH3 in coordination sphere to form complex cation [Co(NH3)6]3+, to make it neutral three NO2

- ligands will come as counter ions to form complex [Co(NH3)6](NO2)3. This complex in water will give four ions: one [Co(NH3)6]3+ and three NO2

- ions. (II) Five NH3 and one NO2

- ligand in coordination sphere to form cationic complex [Co(NH3)5NO2]2+, to make it neutral two NO2

- ligands will come as counter ions to form the complex [Co(NH3)5NO2](NO2)2. This complex in water will give three ions: one [Co(NH3)5NO2]2+

and two NO2- ions.

(III) Four NH3 and two NO2- ligand in coordination sphere to form cationic complex

[Co(NH3)4(NO2)2]+, to make it neutral one NO2- ligands will come as counter ions to form

complex [Co(NH3)4(NO2)2]NO2. This complex in water will give two ions: [Co(NH3)4(NO2)2]+

and one NO2- ions.

(IV) Three NH3 and three NO2 ligand in coordination sphere to form neutral complex, [Co(NH3)3(NO2)3]. This complex in water will not give any ions. Thus the answer for the following question is: Answer (a) [Co(NH3)4(NO2)2]NO2 (b) [Co(NH3)5NO2](NO2)2 (c) [Co(NH3)6](NO2)3

2. Determine the formulas for all possible coordination isomers of the compound [Co(NH3)6][Cr(NO2)6]. Solution: There are four isomers possible by the exchange of NH3 and NO2 ligand: [Co(NH3)5(NO2)][Cr(NO2)5(NH3)] [Co(NH3)4(NO2)2][Cr(NO2)4(NH3)2] [Cr(NH3)5(NO2)][Co(NO2)5(NH3)]

____________________________________________________________________________________________________

CHEMISTRY


[Cr(NH3)4(NO2)2][Co(NO2)4(NH3)2] The compound [Co(NH3)3(NO2)3][Cr(NO2)3(NH3)3] is also possible, however +3 is the most common oxidation state for Cr and Co, therefore it will be a charge neutral species in both cases thus, no interaction between the two. 3. Determine the formulas for all possible polymerization isomers of the compound having empirical formula, Fe(H2O)3(CN)3: Solution: Five possible isomers:

[Fe(H2O)3(CN2)3] (n = 1)

[Fe(H2O)4(CN)2]+[Fe(H2O)2(CN)4]− ( n = 2)

[Fe(H2O)6]3+ [Fe(CN)6]3− ( n = 2)

[Fe(H2O)5(CN)]2+[Fe(H2O)2(CN)4]2−(n= 3)

[Fe(H2O)6]3+[Fe(H2O)2(CN)4]3− (n= 4)

5. Stereoisomerism

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

1. Geometrical isomerism

2. Optical isomerism

5.1 Geometrical Isomerism

Geometrical Isomers differ in the geometric arrangements of the ligands about central metal ion.Geometrical isomers are possible for both square planar and octahedral complexes, but not for tetrahedral complexes. Two types of geometrical isomerism is possible for coordination compounds.

1. cis–trans isomerism

____________________________________________________________________________________________________

CHEMISTRY


2.Facial–meridional isomerism

5.2 Optical isomerism-Optical isomers are the stereoisomers that are non-superimposable mirror images of each other and differ in the direction with which they rotate plane-polarized light. They are said to be “chiral” (handed) and referred to as the enantiomers or enantiomorphs of each other.

6. Summary

In this module, we discussed

• Isomerism in coordination complexes • In general, two types of isomerism are possiblein coordination complexes:

1. Constitutional / Structural Isomerism;and 2. Stereoisomerism • Constitutional / Structural Isomerism is further subdivided in seven types:(a)

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

• Stereo-isomerismis further subdivided in two types:(a) Geometrical isomerism and (b) Optical isomerism

• Constitutional / Structural Isomers have very different physical and chemical properties.

Documents

CHEMISTRY PAPER No.7: Electronic Spectra and …epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/chemistry/... · Electronic Spectra and Magnetic Properties of ... For this complex,