# Structural Isomers in Inorganic Molecules

There are several types of this isomerism frequently encountered in coordination chemistry and the following represents some of them. Isomers that contain the same number of atoms of each kind but differ in which atoms are bonded to one another are called structural isomers, which differ in structure or bond type. For inorganic complexes, there are three types of structural isomers: ionization, coordination and linkage.

### Introduction

Structural isomers, as their name implies, differ in their structure or bonding, which are separate from stereoisomers  that differ in the spatial arrangement of the ligands are attached, but still have the bonding properties. The different chemical formulas in structural isomers are caused either by a difference in what ligands are bonded to the central atoms or how the individual ligands are bonded to the central atoms. When determining a structural isomer, you look at:

1. the ligands that are bonded to the central metal, and
2. which atom of the ligands attach to the central metal.

Below is a quick look at the different types of structural isomers. The highlighted ions are the ions that switch or change somehow to make the type of structural isomer it is.

 Structural Isomer Example Ionization [CoBr(H2O)5]+Cl-  and [CoCl(H2O)5]+Br- Coordination [Zn(NH3)4]+[CuCl4]-2 and [Cu(NH3)4]+[ZnCl4]-2 Linkage [Co(NO2)6]-3 and [Co(ONO)6]-3

### Ionization Isomerism

Ionization isomers are identical except for a ligand has exchanging 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. A hydrate isomer is a specific kind of ionization isomer where a water molecule is one of the molecules that exchanges places.

Example 1: Ionization Isomerism

We have pentaaquabromocobaltate(II)chloride which changes to pentaaquachlorocobaltate(II)bromide.

onization isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.I

one isomer [PtBr(NH3)3]NO2 -> NO2- anions in solution
another isomer [Pt(NO2)(NH3)3]Br -> Br- anions in solution

Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. 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.

• $$[CrCl_2(H_2O)_4]Cl \cdot 2H_2O$$: bright-green colored
• $$[CrCl(H_2O)_5]Cl_2 \cdot H_2O$$: grey-green colored
• $$[Cr(H_2O)_6]Cl_3$$: violet colored

These isomers have very different chemical properties and on reaction with $$AgNO_3$$ to test for $$Cl^-$$ ions, would find 1, 2, and 3 $$Cl^-$$ ions in solution respectively.

### Coordination 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.

Example 2: Coordination Isomerism

Another example

• one isomer [Co(NH3)6] [Cr(C2O4)3]
• another isomer [Co(C2O4)3] [Cr(NH3)6]

Linkage isomerism occurs with ambidentate ligands that are capable of coordinating in more than one way. The best known cases involve the monodentate ligands: SCN- / NCS- and NO2- / ONO-. The only difference is what atoms the molecular ligands use to attach to the central ion. The ligand(s) must have more than one donor atom, but bind to ion in only one place. For example, the (NO2-) ion is a ligand can bind to the central atom through the nitrogen or the oxygen atom, but cannot bind to the central atom with both oxygen and nitrogen at once, in which case it would be called a polydentate rather than an ambidentate ligand. The formula of the complex is unchanged, but the properties of the complex may differ. The names used to specify the changed ligands are changed as well. For example, the (NO2-) ion is called nitro when it binds with the N atom and is called nitrito when it binds with the O atom.

The cationic cobalt complex [Co(NH3)5(NO2)]Cl2 exists in two separable linkage isomers of the complex ion: (NH3)5(NO2)]2+.

(left) The nitro isomer (Co-NO2) and (right) the nitrito isomer (Co-ONO)

When donation is from nitrogen to a metal center, the complex is known as a nitro- complex and when donation is from one oxygen to a metal center, the complex is known as a nitrito- complex. An alternative formula structure to emphasize the different coordinate covalent bond for the two isomers

• $$[Co(ONO)(NH_3)_5]Cl$$: the nitrito isomer -O attached
• $$[Co(NO_2)(NH_3)_5]Cl$$: the nitro isomer - N attached.

Typical ligands that give rise to linkage isomers are:

• thiocyanate, SCN – isothiocyanate, NCS
• selenocyanate, SeCN – isoselenocyanate, NCSe
• nitrite, NO2
• sulfite, SO32−

### References

1. Petrucci, Harwood, Herring, and Madura. General Chemistry: Principles and Modern Applications: Ninth Edition. New Jersey: Pearson, 2007.
2. Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight: Fourth Edition. W. H. Freeman and Company, 2008. Print.

### Problems

1. Write the Coordination Isomer for:   [Co(NH3)6][Cr(CN)6]
2. Write the corresponding linkage isomer as well as names of the two complexes for:   [CoCl(NO2)(NH3)4Cl]
3. What is the coordination isomer of:   [Cr(NH3)6][Fe(CN)6]
4. Write the Ionization isomer for:   [CoBr(NH3)5]SO4
5. Explain a polydentate ligand.

1. [Cr(NH3)6][Co(CN)6]
2. [CoCl(ONO)(NH3)4Cl]
3. [Fe(NH3)6][Cr(CN)6]
4. [CoSO4(NH3)5]Br
5. A polydentate ligand is a ligand that can bind to the central atom of a complex compound at many places at one time.

### Contributors

• The Department of Chemistry, University of the West Indies)

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One kind of isomerism consists of two compounds that have the same empirical formula but differ in the number of formula units present in the molecular formula. An example in coordination compounds is two compounds with the empirical formula Pt(NH3)2Cl2. One is a simple square planar platinum(II) complex, Pt(NH3)2Cl2, and the other is an ionic compound that contains the [Pt(NH3)4]2+ cation and the [PtCl4]2− anion, [Pt(NH3)4][PtCl4]. As you might expect, these compounds have very different physical and chemical properties. One arrangement of the Cl− and NH3 ligands around the platinum ion in the former gives the anticancer drug cisplatin, whereas the other arrangement has no known biomedical applications.
Posted 12:38, 23 Apr 2014
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11:26, 27 Apr 2014

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