CHAPTER - 6

ISOMERISM IN TRANSITION METAL COMPLEXES

NOTES

Isomers: Isomers are different chemical species that have the same molecular formula but different structural formula.

Isomerism, the existence of molecules that have the same numbers of the same kinds of atoms (and hence the same formula) but differ in chemical and physical properties.

Transition elements: d-block elements are known as transition elements. They form complexes due to the availability of vacant (n-1) d orbitals, high charge density and variable oxidation states. Transition elements shown to possess coordination or metal complexes in which metal is at the center and surrounded by the ligands. Ligands are the point charges whose negative end is pointed towards the positive end of metal.

Example of coordination complex

STRUCTURAL ISOMERISM

The complexes which differ in the arrangement of ligands. It is of 7 types.

IONISATION ISOMERISM HYDRATE ISOMERISM LINKAGE ISOMERISM CO-ORDINATION ISOMERISM CO-ORDINATION POSITION ISOMERISM LIGAND ISOMERISM POLYMERISATION ISOMERISM

1. IONISATION ISOMERISM: Ionization isomers are identical except for a ligand has exchanged places with an anion or neutral molecule that was originally outside the coordination complex. The central ion and the other ligands are identical. For example, an octahedral isomer will have five ligands that are identical, but the sixth will differ. The non-matching ligand in one compound will be outside of the coordination sphere of the other compound. Because the anion or molecule outside the coordination sphere is different, the chemical properties of these isomers is different.

For example,

when pentaaquabromocobaltate (II) chloride is dissolved in water, Cl−Cl− ions are generated:

CoBr (H2O)5Cl(s)→CoBr (H2O)+5(aq)+Cl+(aq)

whereas when pentaaquachlorocobaltate(II)bromide is dissolved, Br−Br− ions are generated:

CoCl(H2O)5Br(s)→CoCl(H2O)+5(aq)+Br+(aq).

2. HYDRATE ISOMERISM: A hydrate isomer is a specific kind of ionization isomer where a water molecule is one of the molecules that exchanges places.A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule (Solvate Isomerism). In the case of water, this is called Hydrate isomerism. The best known example of this occurs for chromium chloride "CrCl3.6H2O" which may contain 4, 5, or 6 coordinated water molecules.

[CrCl2(H2O)4]Cl⋅2H2O[CrCl2(H2O)4]Cl⋅2H2O : bright-green colored [CrCl(H2O)5]Cl2⋅H2O[CrCl(H2O)5]Cl2⋅H2O : grey-green colored [Cr(H2O)6]Cl3[Cr(H2O)6]Cl3 : violet colored

3. LINKAGE ISOMERISM

Linkage isomerism is the existence of coordination compounds that have the same composition differing with the connectivity of the metal to a ligand.

Typical ligands that give rise to linkage isomers are:

thiocyanate, SCN− – isothiocyanate, NCS−

selenocyanate, SeCN− – isoselenocyanate, NCSe−

nitrite, NO2−

sulfite, SO32−

Examples of linkage isomers

violet-colored [(NH3)5Co-SCN]2+ and orange-colored [(NH3)5Co-NCS]2+. [FeCl5NO2] and [FeCl5 ONO]

4. CO-ORDINATION ISOMERISM

Coordination isomerism occurs compounds containing complex anionic and cationic parts can be thought of as occurring by interchange of some ligands from the cationic part to the anionic part. Hence, there are two complex compounds bound together, one with a negative charge and the other with a positive charge. In coordination isomers, the anion and cation complexes of a coordination compound exchange one or more ligands. For example, the [Co(NH3)6][Cr(C2O4)3][Co(NH3)6][Cr(C2O4)3] compounds and [Co(C2O4)3][Cr(NH3)6][Co(C2O4)3][Cr(NH3)6] compound are coordination isomers.

5. CO-ORDINATION POSITION ISOMERISM: It occurs due to the difference in the distribution of ligands in two coordination centres. It generally occurs in bridged complexes.

Unsymmertical complexes

Symmertical complexes

6. LIGAND ISOMERISM: is a type of structural isomerism which arises from the presence of ligands which can adopt different isomeric forms. The position isomer of ligand gives ligand isomerism. This isomerism is a special type of isomerism.

7. POLYMERISATION ISOMERISM: Polymerization isomers are those complexes that have the same empirical formula, but differ in molecular mass by integral multiples of empirical mass.

[Pt(NH3)2Cl2]4 and [Pt(NH3)4]or [Pt(Cl)4]

GEOMETRICAL ISOMERISM

It is also known as Cis-Trans isomemrism. Geometrical isomers are such isomers which have same structural formula but differ in the relative arrangement of atoms or groups around the double bond. Metal complexes that differ only in which ligands are adjacent to one another (cis) or directly across from one another (trans) in the coordination sphere of the metal are called geometrical isomers. Geometric isomerism occurs due to the restricted rotation about carbon-carbon double bonds or carbon-carbon single bonds in cyclic compounds

For example:1-butene

This type of isomerism is shown by the Octahedral and square planar complexes.

Sqaure Planar complexes:

Coordination number is 4

MA3B type

They are most important for square planar and octahedral complexes.

Because all vertices of a square are equivalent, it does not matter which vertex is occupied by the ligand B in a square planar MA3B complex; hence only a single geometrical isomer is possible in this case (and in the analogous MAB3 case). All four structures shown here are chemically identical because they can be superimposed simply by rotating the complex in space:

MA2B2 type

For an MA2B2 complex, there are two possible isomers: either the A ligands can be adjacent to one another (cis), in which case the B ligands must also be cis, or the A ligands can be across from one another (trans), in which case the B ligands must also be trans. Even though it is possible to draw the cis isomer in four different ways and the trans isomer in two different ways, all members of each set are chemically equivalent:

Because there is no way to convert the cis structure to the trans by rotating or flipping the molecule in space, they are fundamentally different arrangements of atoms in space. Probably the best-known examples of cis and trans isomers of an MA2B2 square planar complex are cis-Pt(NH3)2Cl2, also known as cis platin, and trans-Pt(NH3)2Cl2, which is actually toxic rather than therapeutic.

[M(AA)2]

en=ethylenediamine (2 bonding sites-coordination number-2)

[MA2BC] type

[MABCD] type

OCTAHEDRAL COMPLEXES

Coordination number is 6

MA4B2 type

If two ligands in an octahedral complex are different from the other four, giving an MA4B2 complex, two isomers are possible. The two B ligands can be cis or trans. Cis- and trans-[Co(NH3)4Cl2]Cl are examples of this type of system:

Pt(NH3)4Cl2]

MA3B3 complex

Replacing another A ligand by B gives an MA3B3 complex for which there are also two possible isomers. In one, the three ligands of each kind occupy opposite triangular faces of the octahedron; this is called the fac isomer (for facial). In the other, the three ligands of each kind lie on what would be the meridian if the complex were viewed as a sphere; this is called the mer isomer (for meridional):

[M(AA)2B2]

[MA2B2C2]

All possible geometrical isomers for the complex [Co(H2O)2(ox)BrCl]−, where ox is −O2CCO2

−, which stands for oxalate.

This complex contains one bidentate ligand (oxalate), which can occupy only adjacent (cis) positions, and four monodentate ligands, two of which are identical (H2O). The easiest way to attack the problem is to go through the various combinations of ligands systematically to determine which ligands can be trans. Thus either the water ligands can be trans to one another or the two halide ligands can be trans to one another, giving the two geometrical isomers shown here:

In addition, two structures are possible in which one of the halides is trans to a water ligand. In the first, the chloride ligand is in the same plane as the oxalate ligand and trans to one of the oxalate oxygens. Exchanging the chloride and bromide ligands gives the other, in which the bromide ligand is in the same plane as the oxalate ligand and trans to one of the oxalate oxygens:

Draw all the possible geometrical isomers for the complex [Cr(en)2(CN)2]+.

Two geometrical isomers are possible: trans and cis.

More Examples:

OPTICAL ISOMERS

Optical isomers are possible for both tetrahedral and octahedral complexes, but not square planar. Optical isomers are stereoisomers that are related via on-superimposable mirror images of each other. They differ from geometric stereoisomers in that they rotate can rotate plane-polarized light. These isomers are referred to as enantiomers or chiral. Optical activity refers to whether or not a compound has optical isomers. A coordinate compound that is optically active has optical isomers and a coordinate compound that is not optically active does not have optical isomers.

OCTAHEDRAL COMPLEXES

The examples you are most likely to need occur in octahedral complexes that contain bidentate ligands, e.g., complex ions like [Ni(NH2CH2CH2NH2)3]2+[Ni(NH2CH2CH2NH2)3]2+ or [Cr(C2O4)3]3−[Cr(C2O4)3]3− . In bidentate complexes or chelating complexes, a ligand binds very tightly to the metal because it holds onto the metal via more than one atom. Ethylenediamine is one example of a bidentate ligand.

M(AA)3 - M(ox)3 Octahedral complex

EXAMPLE: Co(en)3

M(AA)2B2 type

[Co(en)2Cl2]

[Co(ox)2Cl2]

[M(AA)2BC]

[Co(en)2NH3Cl]

MORE EXAMPLES: all optical isomers of [Co(en)4Cl2]

TETRAHEDRAL COMPLEXES

[M(ABCD)]

1.

2.