Isomerism in Coordination Compounds

The transition metals combine to form a large number of complex compounds in which metal atoms are bound to a number of anions or neutral molecules via electron sharing. In modern language, such compounds are referred to as coordination compounds. The chemistry of coordination compounds is very important in modern inorganic chemistry. Chlorophyll, haemoglobin, and vitamin B12 are magnesium, iron, and cobalt coordination compounds, respectively. Coordination compounds are used in a variety of metallurgical processes, industrial catalysts, and analytical reagents. Coordination compounds are also used extensively in electroplating, textile dyeing, and medicinal chemistry.

Some examples of Coordinate compounds are:

• Tetracarbonylnickel (0) – [Ni(CO)₄]
• Potassium tetrahydroxidozincate(II) – K₂[Zn(OH)₄]
• Pentaamminecarbonatocobalt(III) chloride – [Co(NH₃)₅(CO₃)]Cl
• Diamminechloridonitrito-N-platinum(II) – [Pt(NH₃)₂Cl(NO₂)]

Isomerism in Coordination Compounds

Isomers are two or more compounds with the same chemical formula but a different atomic arrangement. They differ in one or more physical or chemical properties due to the different arrangements of atoms. Among coordination compounds, there are two major types of isomerism.

1. Stereoisomerism
2. Structural isomerism

Stereoisomerism

Stereoisomers have the same chemical formula and chemical bonds, but their spatial arrangement is different.

• Geometrical isomerism

Because of the various geometric arrangements of the ligands, this type of isomerism occurs in heteroleptic complexes. Coordination numbers 4 and 6 provide notable examples of this behavior. The two ligands X in a square planar complex of formula [MX₂L₂] (X and L are unidentate) can be arranged adjacent to each other in a cis isomer or opposite to each other in a trans isomer.

Other square planar complexes of the type MABXL (where M is metal and A, B, X, L are unidentified ligands) exhibit three isomers-two cis and one trans. Such isomerism is not possible for a tetrahedral geometry but similar behavior is possible in octahedral complexes of formula [MX₂L₄] ] in which the two ligands X may be oriented cis or trans to each other.

This type of isomerism is also known as geometrical isomerism when didentate ligands L – L [e.g., NH₂CH₂CH₂NH₂ (en)] are present in complexes of the formula [MX₂ (L– L)₂].

Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma₃b₃] like [Co(NH₃ )₃ (NO₂ )₃ ]. The facial (fac) isomer is formed when three donor atoms of the same ligand occupy adjacent positions at the corners of an octahedral face. The meridional (mer) isomer is obtained when the positions are centered on the octahedron’s meridian.

• Optical isomerism :

Mirror images that cannot be superimposed on one another are optical isomers. These are referred to as enantiomers. Chiral molecules or ions are those that cannot be superimposed. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarized light in a polarimeter (d rotates to the right, l to the left). In octahedral complexes containing didentate ligands, optical isomerism is common.

In a coordination entity of the type [PtCl₂ (en)₂ ]²⁺, only the cis-isomer shows optical activity.

Structural isomerism

• Linkage isomerism : 

In a coordination compound containing an ambidentate ligand, linkage isomerism occurs. Complexes containing the thiocyanate ligand, NCS–, provide a simple example, as it can bind through the nitrogen to give M–NCS or through the Sulphur to give M–SCN. This type of behavior is shown in the complex [Co(NH₃ )₅ (NO₂ )]Cl₂ , which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO₂ ).

Examples: 

1. violet-colored [(NH₃)₅Co-SCN]²⁺ and orange-colored [(NH₃)₅Co-NCS]²⁺
2. [Co(ONO)(NH₃)₅]Cl : the nitrito isomer -O attached  and [Co(NO₂)(NH₃)₅]Cl : the nitro isomer – N attached

• Coordination isomerism :

The interchange of ligands between cationic and anionic entities of different metal ions present in a complex causes coordination isomerism. An example is provided by [Co(NH₃ )₆ ][Cr(CN)₆ ], in which the NH3 ligands are bound to Co³⁺ and the CN– ligands to Cr³⁺. In its coordination isomer [Cr(NH₃ )₆ ][Co(CN)₆ ], the NH₃ ligands are bound to Cr³⁺ and the CN^– ligands to Co³⁺ .

Examples:

1. [Co(NH₃)₆][Cr(C₂O₄)₃ ] and [Co(C₂O₄)₃][Cr(NH₃)₆]
2. [Zn(NH₃)₄][Cu(Cl₄)]  and [Cu(NH₃)₄][Zn(Cl₄)]

• Ionisation isomerism :

Ionisation isomerism occurs when the counter ion in a complex salt is a potential ligand in itself and can displace a ligand, which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH₃ )₅ (SO₄ )]Br and [Co(NH₃ )₅Br]SO4 .

Examples:

1. [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅(SO₄)]Cl
2. [CoBr(H₂O)₅]⁺Cl^– and [CoCl(H₂O)₅]⁺Br^–

• Solvate isomerism :

Solvate isomerism is known as ‘hydrate isomerism’ in this isomerism water is involved as a solvent. This is similar to ionisation isomerism. The difference between sulphate isomers is whether the solvent molecule is directly bonded to the metal ion or is present in the crystal lattice as free solvent molecules. The aqua complex [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂.H₂O are two examples (grey-green).

Examples:

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

Solved Question:

Question 1: Draw all geometrical isomers of [Fe(NH₃ )₂ (CN)₄ ]^– .

Answer:

Question 2: Indicate the types of isomerism exhibited by the following complexes  

1. [Co(en)₃ ]Cl₃  
2. [Co(NH₃ )₅ (NO₂ )](NO₃ )₂ 
3. [Pt(NH₃ )(H₂O)Cl₂ ]

Answer:

1. [Co(en)₃ ]Cl₃                                —-> Optical isomerism
2. [Co(NH₃ )₅ (NO₂ )](NO₃ )₂         —-> Optical isomerism, Linkage isomerism, Ionization isomerism
3. [Pt(NH₃ )(H₂O)Cl₂ ]                    —-> Geometrical (cis-, trans-) isomerism

Question 3: How many geometrical isomer will possible in [Co(NH₃)₃(Cl)₃]?

Answer:

Two geometrical isomer is possible for [Co(NH₃)₃(Cl)₃]^–

Question 4: What are ligands in coordination compounds?

Answer:

Ligands are ions or molecules that are bound to the central atom/ion of the coordination entity. Simple ions like Cl–, small molecules like H2O or NH3, larger molecules like H₂NCH₂CH₂NH₂ or N(CH₂CH₂NH₂)₃ , and even macromolecules like proteins can fall into this category.

Question 5: Draw the geometrical isomers of [Pt(NH₃)(Br)(Cl)(py)] ?

Answer: