Prepared by Dr. Hoda El-Ghamry Lecturer of Inorganic Chemistry Faculty of Science-Chemistry Department Tanta University Inorganic Chemistry (2)

Isomerism in inorganic compounds Isomerism: the phenomena in which the compounds have the same chemical composition but different structural arrangement of their atoms isomerism Structural isomerism Ionization isomerism Hydrate isomerism Ligand isomerism Linkage isomerism Coordination isomerism Coordination position isomerism Stereoisomerism (space isomerism)

Stereoisomerism Position or geometrical isomerism Optical or mirror-image isomerism Structural isomerism: This isomerism arises due to the difference in structures of coordination compounds. i) Ionization isomerism: Complexes which have the same emperical formula but give different ions in solution or ionization are called ionization isomerism Example: [Co III (NH 3 ) 5 Br] SO 4 (I), [Co III (NH 3 ) 5 SO 4 ] Br (II) are ionization isomers (I)+BaCl 2 →BaSO 4 (white ppt), (II) +BaCl 2 →no ppt (I)+AgNO 3 → no ppt, (II) +AgNO 3 →yellow ppt

ii) Hydrate isomerism: There are 3 isomers of [Cr(H 2 O) 6 ]Cl 3 1- [Cr III (H 2 O) 6 ] Cl 3 Colour is violet and all the chlorine are precipitated with AgNo 3 2- [Cr III (H 2 O) 5 Cl ] Cl 2.H 2 O Colour is green and two thirds of chlorine are precipitated with AgNo 3 3- [Cr III (H 2 O) 4 Cl 2 ] Cl. 2H 2 O Colour is green and one third of chlorine are precipitated with AgNo 3 Example 2: [Co III (NH 3 ) 4 Cl 2 ] NO 2 (I), [Co III (NH 3 ) 4 (Cl )(NO 2 )]Cl (II) iii) Ligand isomerism: Some ligands themselves are capable of existing as isomers Example:

ortho meta para iv) Linkage isomerism: This type of isomerism is found in those complexes in which the ligands can coordinate with the central metal ion through either of two atoms.

[Co III (NH 3 ) 5 (NO 2 )] 2+ M-NO 2 Nitro (-NO 2 )NO 2 - [Co III (NH 3 ) 5 ONO] 2+ M-ON=ONitrito (-ONO) [Pd II (diph) (SCN) 2 ] M-SCNThiocyanatoNCS - [Pd II (diph) (NCS) 2 ] M-NCSIsothiocyanato [Co III (NH 3 ) 5 SSO 3 ] 2+ M-SSO 3 Thiosulphate-SS 2 O [Co III (NH 3 ) 5 OSO 2 S] 2+ M-OSO 2 SThiosulphate-O Members of each pair of complex ions given above are linkage isomers of each other. Example 1: Example 2: Red in color and easily dissociated with acid Yellow in color and easily not dissociated with acid

v) Coordination isomerism: If both cation and anion of a complex compound are complex, there may be an exchange of ligands between the two coordination spheres giving rise to isomers known as coordination isomers. Types of coordination isomerism: a) Coordination compounds having similar central metal ions in both complex cation and complex anion. Example: [Cr III (NH 3 ) 6 ] 3+ [Cr III (SCN) 6 ] 3-, [Cr III (NH 3 ) 4 (SCN) 2 ] + [Cr III (SCN) 4 (NH 3 ) 2 ] - b) Coordination compounds having different central metal ions in both complex cation and complex anion. Example: [Co III (NH 3 ) 6 ] 3+ [Cr III (SCN) 6 ] 3-, [Cr III (NH 3 ) 6 ] 3+ [Co III (SCN) 6 ] 3-

vi) Coordination position isomerism: In some of the polynuclear complexes, the interchange the ligands between the metal nuclei which are present as a part of the complex give rise to coordination position isomers Example:

Stereoisomerism or space isomerism: When two compounds contain the same ligands coordinated to the same central metal ion, but the arrangement of ligands in space is different, the two compounds are said to be stereoisomers and the type of isomerism is called stereoisomerism. Stereoisomerism geometrical isomerism Optical or mirror-image isomerism 1) Geometrical isomerism(Cis-Trans): Geometrical isomers have identical empirical formula but differ in chemical and physical properties because of the different arrangement of ligands. They are easily separated by different physical and chemical means.

In Cis isomer, two identical (similar) groups are adjacent to each other (Cis=same) In Trans isomer, the two identical groups are diametrically opposite to each other (Trans=across) i.e. the straight line connecting the two groups trans to each other passes through the center at which the metal ion is placed. This type of isomerism is found in square planar and octahedral complexes but not found in tetrahedral complexes …….. Why??????????? 1)Square planar complexes [Ma 4 ], [Ma 3 b], [Mab 3 ] type of square planar complexes don't show this type of isomerism but the following types show this isomerism a)Ma 2 b 2 type complexes Where M is the central metal ion and (a), (b) are monodentate ligands

b) Ma 2 b c type complexes Where M is the central metal ion a, b are monodentate ligands

c) [M(AB) 2 ] n type complexes Where M is the central metal ion and AB represents an unsymmetrical bidentate ligands. A and B represents the two ends (coordination atoms) of the bidentate ligand d) [M(AA) 2 ] n type complexes Where M is the central metal ion and AA represents symmetrical bidentate ligands. e) Bridged binuclear planar complexes of the type [M 2 a 2 b 4 ] n In this type of complexes, Cis, trans in addition to unsymmetrical complexes are possible.

2) Octahedral complexes The arrangement of the six ligands in a regular octahedral complex around the metal center is represented as shown [Ma 5 b] and [Ma 6 ] type of octahedral complexes don't show this type of isomerism But the following types show this isomerism 1) Octahedral complexes containing only monodentate ligands a) [Ma 4 b 2 ] n type complexes In the Cis isomer: The two b ligands have adjacent positions (i.e. they lie on any of the twelve edges of the octahedron In the Trans isomer: The two b ligands are diagonally opposite to each other (i.e. the straight line connecting the b ligands trans to each other passes through the center where the metal ion is placed

a)[Ma 3 b 3 ] n type complexes In the Cis isomer: The three (a) ligands occupy 1, 2 and 3 positions In the Trans isomer: The three (a) ligands occupy 1, 2 and 6 positions 2) Octahedral complexes containing monodentate and symmetrical bidentate ligands a) [M(AA) 2 b 2 ] n type complexes In the Cis isomer: The two (b) ligands are Cis to each other In the Trans isomer: The two (b) ligands are Trans to each other

b) [M(AA) 2 ab ] n type complexes In the Cis isomer: (a) and (b) ligands are Cis to each other In the Trans isomer: (a) and (b) ligands are Trans to each other

2) Optical isomerism This kind of isomerism occurs when a component can be represented by two asymmetrical structures, one of them is the mirror image of the other. a) [M(AA) 2 b 2 ] n type complexes Example: [Co III (en) 2 Cl 2 ] + Mirror

b) [M(AA) 3 ] n type complexes Example: [Fe(ox) 3 ] 3- Mirror