Coordination compounds

Isomerism in Coordination Compounds

2.Stereo (space) isomerism

Stereo isomerism is exhibited by those compounds which have the same position of atoms or groups but these atoms or groups have different arrangement round the central atom. The compounds showing stereo isomerism are called stereo isomers.Stereo isomerism may be of two types viz. geometrical (or cis-trans) isomerism and optical (or d-l or mirror image) isomerism.

i) Geometrical isomerism

The complex compounds which have the same ligands in the coordination sphere but the relative position of the ligands round the central metal atom is different are called geometrical isomers and the phenomenon is called geometrical isomerism.

In a given complex compound the two ligands may occupy positions either adjacent to each other or opposite to each other. The complex compound having two ligands occupying the adjacent positions to each other is called cis-isomer while that in which the ligands occupy opposite positions is called trans-isomer. Thus geometrical isomerism is also called cis-trans isomerism.

Geometrical isomerism is not found in complex compound with coordination number 2 and 3, since in these cases all the positions occupied by the ligands round the central metal atom are adjacent to one another. Geometrical isomerism is most common with the complex comounds having coordination number 4 and 6.

Geometrical isomerism in 4-coordinated complex compounds

Complexes having central atom with 4-coordination number may have either tetrahedral or square planar geometry.

Geometrical isomerism in tetrahedral complexes

Geometrical isomerism cannot be shown by tetrahedral complexes, since all the four ligands n this geometry have adjacent position (i.e., cis position) to one another and all four bond angles are the same (= 109.5°).

Geometrical isomerism in square planar complexes

Before, discussing the geometrical isomerism exhibited by various types of square planar complexes, we should know how cis and trans isomers of a square planar complex are named. A square planar complex having similar ligands at adjacent positions (90° apart) is called cis-isomer while a square planar complex having two similar ligands at opposite positions (180° apart) is called trans-isomer. Thus a square planar complex having two similar ligands at 1-2, 2-3, 3-4 and 1-4 positions is called cis-isomer while that having two similar ligands at 1-3 and 2-4 positions is called trans-isomer. cis- and trans-isomers are also named by numbering system Thus in [Pd²⁺Cl₂BrI]²⁺, if two Cl^- ions are placed cs to each other or at 1-2 postions, it is named as cis-dichloro bromoiodo palladium (II) ion or 1,2-dichloro bormoiodo palladium (II) ion. On the other hand, if two Cl^- ions are placed trans to each other or at 1-3 positions, it is named as trans-dichlorobromoiodo palladium (II) ion or 1, 3-dichlorobromoiodopalladium (II) ion.

Here we shall consider geometrical isomerism in square planar complexes of [Ma₄], [Ma₃b], [Mab₃], [Ma₂b₂], [Ma₂bc], [Mabcd] and [M(AB)₂] type in which M is the central metal atom; a, b, c and d are monodentate ligands and (AB) is an unsymmetrical bidentate ligand, since it has two different donor atoms namely A and B..

1. [Ma₄], [Ma₃b] and [Mab₃] type complexes

Square planar complexes of this type donot show geometrical isomerism, since all the possible spatial arrangement of four ligands round the central metal atom is the same.

2. [Ma₂b₂] type complexes

Important examples of square planar complexes of this type are [Pt(NH₃)₂Cl₂]⁰, [Pt(py)₂Cl₂]⁰, [Pt²⁺(NH₃)₂(NO₂)₂]^o etc. These complexes exist in cis- and trans-isomers. These isomers of [Pt²⁺(NH₃)₂Cl₂]⁰ are shown below. In (a) since both NH₃ molecules and both Cl^- ions are cis to each other, it is called cis-isomer. On the other hand, in (b) since both NH₃ molecules and both Cl^- ions are trans with respect to each other, it is called trans-isomer.

3. [Mabcd] type complexes

Square planar complexes of this type exist in three isomeric forms. For example [Pt²⁺(NH₃)(py)(Cl)(Br)] exists in three isomeric forms shown below in figure. These isomeric forms can be obtained by selecting one ligand, say NH₃, and then placing the remmaining three ligands, one by one, trans toNH_3.

The existence of three isomeric forms in case of the complexes mentioned above indicates that these complexes have square planar geometry.

4. [Ma₂bc] type complexes

Square planar complexes of this type also show cis-trans isomerism. For example [Pt²⁺(NH₃)₂NO₂Cl]^2-ion exists in cis and trans-isomers as shown below in the figure.

5. [M(AB)₂ type complexes

Here M is the central metal ion and [AB] represents an unsymmetrical bidentate ligand in which A and B are two different coordinating (donor) atoms. Square planar complexes of this type also exist in cis- and trans-isomers. For example [Pt²⁺(gly)₂] exists in cis- and trans- isomers are shown below. Here gly represents the glycinato ligand, NH₂.CH₂COO^- which has N and O atoms as its donor atoms (i.e, A = N and B = O).

[Cu²⁺(gly)₂]⁰ ia also an example of square planar complex of [M(AA)₂] type. This complex also exhibits cis-trans-isomerism.

Geometrical isomerism in 6-coordinated complexes : Octahedral complexes

We know that a complex compound havng central atom with coordination number equal to 6 is octahedral in shape. Before discussing the geometrical isomerism exhibited by various types of octahedral complexes.

Octahedral complexes of type [Ma₆], [Ma₅b] do not exhibit geometrical isomerism.

1. [Ma₄b₂] type complexes

Important examples of octahedral complexes of this type are [Co³⁺(NH₃)₄Cl₂]⁺,[Co³⁺(NH₃)₄(NO₂)₂]⁺etc. These complexes exist in cis and trans-somers. These isomers of [Co³⁺(NH₃)₄Cl₂]⁺ion are shown below. In cis-isomer two Cl^- ions occupy the adjacent (i.e., 1 and 2) positions of the octahedron while in trans-isomers these ions have the oposite (i.e., 1 and 6) position. Cis-isomer of[Co³⁺(NH₃)₄(NO₂)₂]⁺ion is yellow-brown while trans-form is yellow in colour.

2. [Ma₃b₃] type complexes

[Co(NH₃)₃Cl₃],[Co(NH₃)₃(NO₂)₃], [Cr(H₂O)₃F₃], etc. are important.

3. [Ma₄bc] type complexes

4. [M(AB)₃] type complexes

Here (AB) represents an unsymmetrical bidentate ligand in which A and B are two different coordinating (donor) atoms. Octahedral complexes of this type exist in cis- and trans-isomes. As an example the cis- and trans-isomers of [Cr³⁺(gly)] has been shown in figure. Each of these forms is optically active and hence each has a pair of optical isomers. In cis-isomer two N-atoms and two O-atoms of two gly ions are placed at cis-positions while in trans-isomer these atoms are occupying trans-positions.

Reference.

F.A.Cotton and Wilkinson G.Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D.Concise Inorganic Chemistry.fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary.A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.