Coordination compounds - Isomerism K.Sridharan Dean School of Chemical & Biotechnology SASTRA University Thanjavur 613 401 Page 1 of 9
Table of Contents 1 Types of isomerism... 3 1.1 Types of isomerism... 3 1.1.1 Hydrate isomerism... 3 1.1.2 Coordination isomerism... 3 1.1.3 Ionization isomerism... 4 1.1.4 Linkage isomerism... 4 1.1.5 Optical isomerism... 4 1.1.6 Cis trans isomerism... 4 1.1.7 fac mer isomerism... 5 2 Coordination number (C.N.) 4 & 5 and isomerism... 6 2.1 C.N 4 and isomerism... 6 2.1.1 Tetrahedral complexes... 6 2.1.2 Square planar complexes... 6 2.2 C.N.5 and isomerism... 7 3 C.N.6 and isomerism... 7 3.1 cis trans isomerism... 7 3.2 fac mer isomerism... 7 3.3 Optical isomerism... 8 3.4 Designation of the isomers... 8 4 References... 9 Page 2 of 9
1 Types of isomerism Isomerism means same molecular formula but different structural formula. 1.1 Types of isomerism 1. Hydrate isomerism 2. Coordination isomerism 3. Ionization isomerism 4. Linkage isomerism 5. Optical isomerism 6. Cis-trans isomerism 7. Fac-mer isomerism 1.1.1 Hydrate isomerism Here, the total number of aquo ligands (water molecules acting as ligands) will be the same but the number of water molecules coordinating to the central metal ion will be different. In other words, the total number of water molecules will be the same while the number present in the coordination sphere will be different from that in the ionization sphere. Examples: [Cr(H 2 O) 6 ]Cl 3 Violet [CrCl(H 2 O) 5 ]Cl 2.H 2 O Blue-green [CrCl 2 (H 2 O) 4 ]Cl.2H 2 O Green 1.1.2 Coordination isomerism This is shown when both the cation and anions are complex ions. Two different metal ions are there having different ligands. Then these ligands are exchanged between the metal ions. Example: [Co(NH3)6][Cr(CN)6] NH3 is attached to Co and CN is attached to Cr [Co(CN)6][Cr(NH3)6] CN is attached to Co and NH3 is attached to Cr Page 3 of 9
1.1.3 Ionization isomerism Here, the ligands present in the coordination and ionization spheres are exchanged. Example: [PtCl 2 (NH 3 ) 4 ]Br 2 ; chloride ions are coordinated to Pt while bromide ions are present outside in the ionization sphere [PtBr 2 (NH 3 ) 4 ]Cl 2 ; bromide ions are coordinated to Pt while chloride ions are present outside in the ionization sphere. 1.1.4 Linkage isomerism Example: Here, the same ligand is attached the central metal ion by different atoms. This is possible only when there are more than one donor atoms present in the ligand. Example: [Co(NO 2 )(NH 3 ) 5 ]Cl 2 ; [Co(ONO)(NH 3 ) 5 ]Cl 2 ; Here, N of NO 2 is linked to the metal Here, O of NO 2 is linked to the metal 1.1.5 Optical isomerism This is due to the difference in the spatial arrangement of groups. The essential condition for this is that the molecules should have nonsuperimposable mirror images. This is possible only when the molecule has no S n axis, that is, rotationreflection axis. These complexes will rotate the plane polarized light either to the right (dextro rotatory) or the left (levo rotatory). 1.1.6 Cis-trans isomerism If similar groups are on the same side, it is called a cis isomer and if they are on the opposite side, it is called a trans isomer. Page 4 of 9
1.1.7 fac-mer isomerism This is common in octahedral complexes. The numbering of positions in an octahedral complex must be understood to understand this isomerism. The numbering is shown below: 1,2,3 isomer is called facial or fac- isomer and 1,2,6 isomer is called meridional or mer- isomer. The numbers represent the positions of the similar ligands. This type of isomerism is possible in M(a3b3) type complexes. Page 5 of 9
2 Coordination number (C.N.) 4 & 5 and isomerism 2.1 C.N 4 and isomerism Two geometries, viz., tetrahedral and square planar are possible in C.N. 4 2.1.1 Tetrahedral complexes Tetrahedral complexes will show only optical isomerism. These are mainly tetrahedral complexes and only a few complexes having four different ligands are synthesized because the ligands are labile, that is, rapidly exchanging. Optical isomerism is possible only when unsymmetrical ligands are present. 2.1.2 Square planar complexes These will show both optical as well as cis-trans isomerism. Fig 2.1.2.1 Square planar complex with plane of symmetry These are not optically active because they have a plane of symmetry, that is, the molecular plane. Fig 2.1.2.2 Square planar complex with no plane of symmetry Page 6 of 9
The square planar complex shown in Figure 2.1.2.2 has no plane of symmetry and hence shows optical activity. 2.2 C.N.5 and isomerism The isomerism shown is SP-TBP isomerism, that is, square pyramidal trigonal bipyramidal isomerism. Example: Chlorobis[1,2-bis(diphenylphosphino)ethane]cobalt(II) cation. The SP complex is red and TBP complex is green in color. 3 C.N.6 and isomerism These complexes show cis-trans, fac-mer, and optical isomerism. 3.1 cis-trans isomerism 3.2 fac-mer isomerism Page 7 of 9
3.3 Optical isomerism This is possible in chelate complexes having unsymmetrical ligands, where Sn axis will not be present. 3.4 Designation of the isomers and Λ have been used to designate absolute configurations of tris chelate octahedral complexes. is used to denote a right-handed spiral about the C3 axis and Λ to denote the left- handed one. Steps: 1. View along the C3 axis 2. Start from a rear atom in the bidentate ligand with your right finger and reach the front atom. 3. If the thumb points towards you, then it is a isomer. 4. If the same thing is possible with a left finger and the left thumb points towards you, then it is a Λ isomer. Page 8 of 9
2 1 N 1 N 2 3 4N N 4 3 C 3 C 3 4 References 1. Inorganic Chemistry: Principles of Structure and Reactivity, James E.Huheey, Ellen A.Keiter, Richard L.Keiter, Okhil K.Medhi, Pearson Education, Delhi, 2006 2. Inorganic Chemistry, Shriver and Atkins, 3/e, Oxford University Press, 2002, 3. Concise Inorganic Chemistry, 5/e, Blackwell Science, 2005, 4. Concepts and Models of Inorganic Chemistry, 3/e, John Wiley & Sons Page 9 of 9