Electronic Spectra and Magnetic Properties of Transition Metal Complexes)

Transcription

1 Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal 22: Isomerism part IV Optical isomerism

2 TABLE OF CONTENTS 1. Learning outcomes 2. Optical isomerism in octahedral complexes 3. Isomerism from ligand distribution and unsymmetrical ligands 3.1 Octahedral complexes with monodentate ligands 3.2 Octahedral complexes with bidentate ligands 4. Isomerism from ligand conformation and chirality 5. Solved problem 6. Physical and chemical properties of optical isomers 7. Summary

3 Prerequisites Before going into the details of this module we should be aware of definition of isomerism 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 one of them can be further subdivided. 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 should be aware of (a) Coordination isomerism, (b) Polymerization isomerism, (c) Ionization isomerism, (d) Hydrate isomerism (e) Linkage isomerism and (f) Ligand isomerism in details. Moreover, we should be aware of geometrical isomerism. We should also be aware of the term optical isomerism and how optical isomerism is possible for tetrahedral as well as square planar metal complexes. In this module we will start our discussion from optical isomerism in octahedral complexes.

4 1. Learning Outcomes After studying this module, you shall be able to Know about optical isomerism in octahedral metal complexes Learn how optical isomerism is possible for octahedral complexes with monodentate ligands Learn how optical isomerism is possible for octahedral complexes with bidentate ligands Identify the structures of possible optical isomers for octahedral complexes with monodentate as well as bidentate ligands Evaluate the number of possible optical isomers for octahedral complexes 2.Optical isomerism in Octahedral Complexes As we know that the most important criterion for the existence of an enantiomer of a molecule is that it should be dissymmetric; that is, it can possess other elements of symmetry but must lack S n axis. There are instances of chirality and optical isomerism that do not depend on having four different groups attached to a tetrahedral central atom. Indeed there are six ways for octahedral complexes to be dissymmetric: a) The distribution of monodentate ligands about the central metal. b) The distribution of chelate rings about the central metal. c) The coordination of unsymmetrical multidentate ligands. d) The conformation of chelate rings. e) The coordination of a chiral ligand. f) The coordination of a donor atom that is asymmetric. The important cases are described below: 3. Isomerism from Ligand Distribution and Unsymmetrical Ligands 3.1 Octahedral Complexes with Monodentate Ligands In an octahedral complex, chiral metal center can be produced by the appropriate arrangement of different types of monodentate ligands. Among numrous ways some of them are discussed below: 1. Octahedral complex, MABCDEF: Octahedral complexes with all six different ligands. There are numerous ways to arrange six ligands around the central metal ion and this complex will have maximum number of stereoisomers possible. In such a system, it is very difficult to find out the number of isomers possible. For this Bailer suggested a method for the isomer enumeration, which is as follows:

5 Table I II III AB AB CF DF DE AB CD EF BD EF AD BC EF BC DF BC DE BE DF AD BE CF BD CF BD BF DE AD BF BF CD BE CD 1. Take one ligand A as a fixed point of reference. 2. Place remaining five ligands in other possible positions. 3. Arrange the ligands in such a way, that AB, CD and EF, make a trans pair (Figure 1). The resulting pair is listed in the first row (1) and first column (I) of the table 1 4. Now to write another isomer, hold one trans pair constant (AB) and hold one of the pair constant in the next row (C), Continue changing second ligand until done (CD), then (), then (CF). The resulting pairs are listed in the column (II) and column (III) of the first row (1). In the above steps, three isomers are generated that are possible: when AB trans pair is fixed and remaining ligands are permuted on the other coordination positions. 5. Now if we replace ligand B by C (Figure 1) and other four ligands (B, D, E and F) are permuted on the other remaining positions, there will be again three isomers generated as listed in the second row (2) of the table 1.

6 Figure 1 6. If we continue the process of replacing ligand trans to A and changing the position of other ligands, will get third, fourth and fifth rows of the table 1. There are five rows and three columns in the table 1. Thus, 15 stereoisomers are generated in this way. Now each isomer listed in table 1 is dissymmetric (Figure 2), but they are not related to each other as mirror images so they are called diastereomers. Diastereoisomers may be chiral or they may be achiral. Thus, this complex will have 15 chiral diastereomers. Now each of the 15 diastereomers, is dissymmetric (Figure 2), will have a nonsuperimposable mirror image means enantiomer. Consequently total 30 stereoisomers are possible for this complex. Figure 2 2. Octahedral complex, MA 2 C 2 EF:There are two sets of identical ligands. The two sets are A (A=B in MABCDEF) and C (C=D in MABCDEF). Only ligands E and F are different. On substituting, A for B and C for D in table 1, will generate table 2. It is clear from the table 2 that there are duplicate configurations. 1(II)= 1(III), 2(I)= 3(I), 2(II)= 3(II), 4(I) and 4(II), 2(III)= 3(III), 5(I) and 5(II), 4(III)= 5(III) The duplicates are indicated in table 2 by red colour. This leaves six distinguished configurations. Out of which two are dissymmetric and have enantiomers.

7 Table I II III AA AA CF CF AA CC EF EF EF CF CF CF CF CC CC For example, [Cr(Br) 2 (NH 3 ) 2 (CH 3 )(H 2 O)] +, will have eight stereoisomers (Figure 3). For drawing stereoisomers we will first fix position of at least two of the ligands attached and then rotate the other ligands one by one. In the complex, [Cr(Br) 2 (NH 3 ) 2 (CH 3 )(H 2 O)] +, we will fix two Br - ligands on trans position and then arrange the other ligands trans to each other, then we can rotate the position of ligands other than Br -. Thereby we get eight stereoisomers, out of them two isomers form enantiomeric pair. Figure 3 3. Octahedral complex, M(A 2 C 2 E 2 ): There are three sets of identical ligands and these three sets are A (A=B in MABCDEF), C (C=D in MABCDEF) and E (E=F in MABCDEF).. On substituting, A for B, C for D and E for F in table 1, will generate table 3. It is clear from the table 3 that there are duplicate configurations. 1(II)= 1(III), 2(I)= 3(I), 2(II)= 2(III), 3(II), 4(I) and 4(II),

8 2(III)= 3(III), 5(I) and 5(II), 4(III)= 5(III) Table I II III AA AA AA CC EE EE EE CC CC The further duplicated configurations are indicated in table 3 by green colour. This leaves five distinguished configurations. Out of which only one isomer (Figure 4, V) is dissymmetric and will have enantiomer. Other four diastereomers (Figure 4, I, II, III and IV) have plane of symmetry. Figure 4 For example, [Co(NH 3 ) 2 (H 2 O) 2 Cl 2 ] has been resolved into its enantiomers I & II (Figure 5). Other configurations for this complex are having plane of symmetryand they are therefore achiral.

9 Figure 5 The number of stereoisomers possible for the given complex with monodentate ligands can be calculated using Bailer s method. By this method possible number of stereoisomers is calculated for different cases and tabulated in table 4 Table 4 Total number of stereoisomers Pair of enantiomers General Formula MA MA 5 B 1 0 MA 4 B MA 3 B MA 4 BC 2 0 MA 3 BCD 5 1 MA 2 BCDE 15 6 MABCDEF MA 2 B 2 C MA 2 B 2 CD 8 2 MA 3 B 2 C Octahedral Complexes with Bidentate Ligands Any octahedral trischelate, whether with a symmetrical bidentate ligand, is chiral and will have optical isomers. Thus, the octahedral complexes containing bidentate or higher chelating ligands can form optical isomers. 1. Octahedral complex, M(A-A) 3 :For example, the tris(ethylenediamine)cobalt(iii) ion (Figure 6) is chiral, in spite of the fact that the three ethylenediamine ligands are identical and are themselves symmetrical. Complexes with three rings, such as [Co(en) 3 ] 3+, can be viewed like a propeller with three blades. The structure can be either left or right handed, with non-superimposable mirror images.

10 Rotate Nonsuperimposable Figure 6 2. Octahedral complex, M(A-A) 2 X 2 :There will be two geometrical isomers possible for this type of complex: cis and trans. cis-m(a-b) 2 X 2 is chiral and will also have optical isomers. On the other hand, trans-m(a-b) 2 X 2 is achiral and will have only one structure. The optical isomers of cis-[co(en) 2 Cl 2 ] + are shown in figure7. The figure 8 describes the structure of trans - [Co(en) 2 Cl 2 ] +. Structures I and II are enantiomers to each other, however the pairs I & III and II & III are diastereoisomers or diastereomers. Figure 7 Figure 8

11 3. Octahedral complex, M(A-B) 2 X 2 :Dissymmetry can be introduced in a octahedral complex by introducing unsymmetrical bidentate ligand to it as well. For example, the complex [Cu(H 2 NCH 2 COO) 2 (H 2 O) 2 ] is chiral (Figure 9). In this complex, glycinate ion can bind to the metal center in two ways; (i) Nitrogen and oxygen of one ligand can be cis; (ii) They can be trans to the nitrogen and oxygen of another ligand. All the possible arrangements can give rise to five diastereomers and out of which three are enantiomers. This means a total of eight stereoisomers are possible. Figure 9 4. Octahedral complex, M(A-B) 3 : Any octahedral tris-chelate, with an unsymmetrical bidentate ligand is chiral and will have optical isomers. Thus, the octahedral complexes containing bidentate or higher chelating ligands can form optical isomers. For example, the tris(glycinato)cobalt(iii) complex, [Co(H 2 NCH 2 COO) 3 ] (Figure 10) is chiral. The nitrogen and oxygen atom of one ligand can be cis or trans to the nitrogen and oxygen atom of other ligand. Thus, four stereoisomers are possible for this complex. Figure 10 The numbers of stereoisomers possible for the given complex with bidentate ligands can be calculated using Bailer s method. By this method possible number of stereoisomers is calculated for different cases and tabulated in table 5

12 Table 5 Total number of stereoisomers Pair of enantiomers General Formula M(A-A)(B-C)DE 10 5 M(A-B) 2 CD 11 5 M(A-B)(C-D)EF M(A-B) Isomerism from Ligand Conformation and Chirality Coordination complexes can be made dissymmetric by attaching a multidentate ligand so that the chelate rings thereby formed can exist in more than one conformation. For example, ethylenediamine bound to a metal ion creates a five membered puckered ring with a two-fold axis of symmetry (Figure 11). Two conformations are clearly possible for the ring; one is labeled as λ and the other as δ. It is also clear from the figure 11, that these conformations are nonsuperimposable mirror images. However, they have little conformational stability (low energy barrier for ring inversion), so it is not possible to isolate two optical isomers of a complex such as [Co(en)(NH 3 ) 4 ] 3+. Figure 11 Another way to introduce chirality into a coordination complex is to use a chiral ligand. For example [Co(NH 3 ) 5 (S-amH)] 3+ (Figure 12) is optically active complex, where S-amH is a zwitterions of S-alanine, which is achiral ligand. Figure Solved problem

13 Question1. For each of the hypothetical complex [Co (en)(cl) (NH 3 ) 2 (H 2 O)], draw structures of all the possible stereoisomers and indicate which one is chiral. Solution:For drawing stereoisomers we first fix position of at least two of the attached ligands and then rotate the other ligand one by one. For the complex [Co(en)(Cl)(NH 3 ) 2 (H 2 O)], we will fix the position of ethylenediamine ligand and rotate the others. Thus, we can get six stereoisomers, out of them two isomers form enantiomeric pair (Figure 13). Figure 13 6.Physical and Chemical Properties of Optical Isomers Unlike geometric isomers, optical isomers (enantiomers) have identical physical and chemical properties, such as: Solubility Boiling point Color Dipole moment Chemical reactivity towards non chiral reagents However, they have different optical properties. That means interaction of enantiomers with plane polarized light is different.

14 7. Summary In this module, we discussed about Optical isomerism in octahedral complexes Enantiomers and diastereomers Enantiomers have identical physical and chemical properties Diastereomers have different chemical and physical properties