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 Complexes) 23 and Isomerism part III Optical isomerism
TABLE OF CONTENTS 1. Learning Outcomes 2. Optical Isomerism 3. Optical Isomerism in Tetrahedral Complexes 3.1 Tetrahedral complexes with four different substituents 3.2 Tetrahedral complexes with unsymmetrical bidentate ligand 4. Optical Isomerism in square planer complexes 4.1 Square planar complexes with chiral bidentate Ligand 4.2 Square planar complexes with achiral bidentate Ligand: Special case 4.3 Square planar complexes with specially chosen bidentate ligands 5. Stereochemical notations for the tetrahedral complexes 6. Summary
Prerequisites Before going into the details of this module we should be aware of definition of isomerism and types of isomerism ie. 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 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. In this module we will start our discussion from optical isomerism.
1. Learning Outcomes After studying this module, you shall be able to Know about optical isomerism Learn how optical isomerism is possible for coordination complexes Identify the structures of possible optical isomers for tetrahedral as well as square planar complexes Evaluate the reasons of optical isomerism in square planar complexes. 2.Optical isomerism Optical isomers are the stereoisomers that are nonsuperimposable mirror images of each other and differ in the direction with which they rotate plane-polarised light. They are said to be chiral (handed) and referred to as enantiomers or enantiomorphs of each other. An object or a system is chiral if it differs from its mirror image, and its mirror image can not be superimposed on the original object. A molecule can be chiral only if it lacks an improper axis of rotation- that is, a rotation-reflection axis (S n ). However, molecule can also be considered as chiral if it doesn t possess a center or plane of symmetry. The later criterion must be applied carefully, because some molecule may lack both center and plane of symmetry but still have superimposable mirror image, means compound is achiral. For example 1,3,5,7- tetramethylcyclooctatetraene (Figure 1) does not possess a centre of inversion or a reflection plane but it is superimposable on its mirror image as it possess an S 4 axis. Figure 1 The fact that a molecule is chiral only if it lacks the all-important S n axis does not mean that they must also be asymmetric. In fact, in coordination chemistry, many optically active compounds have been isolated having proper rotation axis. For example tris (bidentate) metal complex (Figure 2) has proper axis rotation. In summary, the most important criterion for 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.
Figure 2 Human hands are perhaps the most universally-recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand (Figure 3); no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. Chirality is sometimes also referred as handedness. It is also found in propellers, spiral staircases and pasta spirals etc. A right-handed (clockwise) twist of a propeller cannot be turned or flipped to look like a left-handed (anticlockwise) propeller unless it is viewed through a mirror, and vice versa. An organic compound is said to be chiral if in that compound, four different groups/atoms are arranged tetrahedrally around the carbon (Figure 4). Optical isomers are usually associated with tetrahedral or octahedral geometries in transition metal complexes. Square planar complexes don t show optical isomerism except for some rare examples. Figure 3 Figure 4 3.Optical isomerism in Tetrahedral Complexes
Tetrahedral complexes can be chiral in the same way that organic compounds are: they may have four different ligands and may also have unsymmetrical chelating ligands. Tetrahedral complexes of the type, MABCD (Figure 5) are analogous to carbon bonded to four different groups, and thus have optical isomers, but not geometric isomers. The optical isomerism is rarely observed in tetrahedral complexes with four different substituents because substituents in these complexes are usually too labile for the complex to be resolved, i.e., they racemize rapidly. 3.1 Tetrahedral Complexes with four different substituents The best examples of tetrahedral complexes with stereogenic metal centers contain organometallic carbon-bonded ligands. In many cases, the description tetrahedral is somewhat spurious as it relates to the number of ligands and not the bonding mode of (and number of formal coordination sites occupied by) the ligands. An interesting example is cyclopentadienyliron phosphine carbonyl complex (Figure6). In these complexes the C 5 H 5 ring forces the ligands back until bond angles are essentially 90 rather than 109.5. Indeed an argument could be made for considering the complex to be eight-coordinate. The chirality of the molecule is the important feature to be noted. Another example is so-called half-sandwich or piano-stool compounds of the type shown in Figure 7 are classical examples of such chiral species, which may often be resolved by classical methods. In the rhenium complex the η 5 - cyclopentadienyl ligand is considered to occupy just one coordination site, resulting in the tetrahedral description. Figure 5 Figure 6 Figure 7 3.2 Tetrahedral Complexes with unsymmetrical bidentate ligand
Another way to observe chirality for tetrahedral complexes is to use unsymmetrical bidentate ligands (not necessarily asymmetric or chiral itself). For example, Ni(II) complex with unsymmetrical β- diketone and β- ketoamine (Figure 8). A very few tetrahedral bis-chelate complexes have been even partially resolved into their enantiomers and most of them involve non-transition elements such as beryllium, boron and zinc. The transition metal complexes that have been partially resolved racemize quite rapidly. Figure 8 Another example of this type of system is bis(benzoylacetonato)beryllium complex (Figure 9); two enantiomers of this complex have been isolated. The analogous complex with ligand acetylacetone, [Be(acac) 2 ] (Figure 10) is not chiral. It is because of the fact that acetylacetone is a symmetrical ligand and benzoylacetone is unsymmetrical. Figure 9 Figure 10 4.Optical isomerism in Square Planer Complexes Square planer complexes are generally achiral since molecular plane of the complex acts as a plane of symmetry. But optical isomerism may also appear in square planar complexes having (i) an asymmetric ligand or (ii) specially chosen bidentate ligands.
4.1Square planar complexes with chiral bidentate Ligand A simple way of introducing chirality in square planer complexes is to use chiral ligand. For example, a platinum(ii) complex (Figure 11) with N-methyl-N-ethylglycine ligand, having asymmetric nitrogen center show optical isomerism. Figure 11 4.2 Square planar complexes with achiral bidentate Ligand: Special case Although it is commonly stated that square-planar complexes with achiral bidentate ligands, do not give enantiomeric complexes, this is not strictly correct. A classic example played a crucial role in the establishment of the square planar geometry for platinum complexes. The cation [Pt(dpen)(mpn)] 2+ (dpen =meso-1,2-diphenylethane-1,2-diamine; mpn = 2-methylpropane-1,2 diamine) contains two achiral ligands. The complex can be resolved into two enantiomers establishing the square-planar geometry, as it would be achiral if the geometry were tetrahedral (Figure 12). Figure 12
4.3 Square planar complexes with specially chosen bidentate ligands For example, a platinum(ii) complex (Figure 7) with the ligand meso-stilbenediamine and one molecule of isobutylenediamine. When this complex is square planar, the molecule (Figure 13) has no improper axis of rotation (S n ) and is therefore chiral. On the other hand if the complex is tetrahedral, a plane of symmetry is clearly present, and is therefore achiral. Thus carefully designed complexes with square planer structures have no improper axis of rotation (S n ) and hence are chiral. Figure 13 5.Stereochemical Notations for the Tetrahedral Complexes The absolute configuration of the chiral tetrahedral complexes with four different substituents is defined by Cahn Ingold Prelog (CIP) rules. Similar for the chiral organic compounds, the chiral tetrahedral complexes are also named as (R) and (S) (Figure 14). The R and S configurations are assigned by a sequence rule considering the priority order of substituents at the chiral center. The substituents or ligands are assigned by decreasing priority in the order of decreasing atomic number and a free electron pair has the lowest order. For example the priority order for the hypothetical chiral organic compound is NH 2 > CHO > CH 3 > H. The priority numbers having been assigned, the stereochemistry of the chiral center is then specified by noting whether the priority descends (1 2 3) in a clockwise (R) or counterclockwise (S) direction when one views the chiral center from the face of the molecule opposite the lowest priority atom. So the first isomer in the figure 14, is R and other is S. As we have already studied, that the tetrahedral complexes with four different substituents are difficult to synthesize or they are very unstable. Only η n -arene complexes (n = 5/6; arene = C 5 H 5, C 6 H 6, C 6 Me 6,and C 10 H 14 ) are the example for this type of system. For η n -arene complexes (n = 5/6; arene = C 5 H 5, C 6 H 6, C 6 Me 6,and C 10 H 14 ) where ligand is bonded to the metal center through more than one atom, absolute configuration of the chiral metal is yet to be recommended by IUPAC.η n -arene ligands are considered to be attached to the metal center through more than one bond but not with the single bond.
Figure 14 Figure 15 Thus, the determination of absolute configuration about a chiral metal center is based on the sum of atomic weights of the atoms in polyhapto (η n -arene) ligands coordinated to the metal center. For example, η 5 -C 5 H 5 would be considered to be a pseudoatom of molecular weight 60 (five carbon atoms bonded to the metal) and the η 6 C 6 H 6 of molecular weight 72 (six carbon atoms bonded to the metal center). In η 5 -C 5 H 5 -Fe(II) complexes the priority is therefore, η 5 C 5 H 5 >Cl> P (such as in PPh 3 ) > N (such as in amines) (Figure 15). So the first isomer is R and other one is S. In η 6 C 6 H 6 -Fe(II) complexes the priority is therefore, η 6 C 6 H 6 >Cl> P (such as in PPh 3 ) > N (such as in amines) (Figure 16). So the first isomer is R and other is S. The CIP rules are also used for defining the absolute configuration of the half sandwich complexes containing more than one stereocenter 2. Figure 16
6. Summary In this module we discussed about Optical isomerism in tetrahedral as well as square planer complexes Chirality in tetrahedral complexes can be introduced by using (i) four different substituents or (ii) unsymmetrical bidentate ligand Chirality in square planar complexes can be introduced by using (i) chiral bidentate ligand or (ii) specially chosen bidentate ligands A special case where, square planer complexes with achiral bidentate ligand is isolated. Stereochemical notations R and S, for the tetrahedral complexes with four different substituents.