A. Introduction to Isomerism and Isomer Classifications.

Isomers: Compounds with the same formula but different properties that result from different structures. There are two broad classes of isomers: structural isomers and stereoisomers.

Structural isomers have the same molecular formula but different molecular structures (different connectivities or different numbers and kinds of chemical bonds.

Organic examples of structural isomers:

• CH₃OCH₃ and CH₃CH₂OH
• cyclobutane and 1-butene

Geometric isomers have different spatial arrangement results in different geometries (different bond angles or different distances between nonbonded atoms, for example).

Organic example: cis- and trans-2-butene, CH₃CH=CHCH₃

Optical isomers have the same geometrical parameters but are related as nonsuperimposable mirror images. (In other words, the molecule or ion is chiral.). Optical isomers get their names because they are able to rotate a plane-polarized light beam to the left or to the right.

Organic example: CHFClBr. A carbon atom with four different groups attached to it has a nonsuperimposable mirror image.

B. Structural Isomers in Transition Metal Complexes

There are many types of structural isomers in transition metal complexes. We will explore three of them.

1. Ionization isomers. Ligands inside the coordination sphere exchange places with ligands outside the coordination sphere. Ionization isomers are so-named because they give different ions when dissolved in water.

Example 1. There are two isomers with the formula PtCl₂SO₄.4NH₃. Solutions of both are neutral, meaning that the ammonia must be coordinated to the platinum(IV) ion. [Pt(NH₃)₄Cl₂]SO₄ and [Pt(NH₃)₄Cl(SO₄)]Cl.

Isomer A does not react with silver nitrate but gives a white precipitate of BaSO₄ when a solution of barium chloride is added to it. Thus chloride is coordinated to platinum but sulfate is not.

Isomer B does not react with barium chloride but gives a white precipitate of AgCl when a solution of silver nitrate is added to it. Thus sulfate is coordinated to platinum but chloride is not.

Thus Isomer A is [Pt(NH₃)₄Cl₂]SO₄ and Isomer B is [Pt(NH₃)₄Cl(SO₄)]Cl (platinum (IV) has a coordination number of 6, so one of the chlorides remains coordinated to the platinum).

Example 2. There are three compounds with the formula CrCl₃.6H₂O. One is violet, one is grey-green, and the third is deep green.

The violet isomer produces 3 moles of silver chloride upon reaction with silver nitrate, and does not lose water in a desiccator.

The grey-green isomer gives 2 moles of silver chloride upon reaction with silver nitrate, and loses one mole of water when stored in a desiccator.

The deep green isomer gives 1 mole of silver chloride upon reaction with silver nitrate, and loses two moles of water when stored in a desiccator.

Thus the three isomers are [Cr(H₂O)₆]Cl₃ (violet), [Cr(H₂O)₅Cl]Cl₂.H₂O (grey-green), and [Cr(H₂O)₄Cl₂]Cl.2H₂O (deep green).

Note that the chloride ions that react with silver nitrate are the ones not bonded to the chromium(III) ion, and the water molecules that are lost in a desiccator are the uncoordinated ones.

The chromium complexes in this example are sometimes said to be an example of hydrate isomerism, a subclass of ionization isomerism in which one or more water molecules exchange with ions.
 
 

2. Linkage isomers. Linkage isomers can exist when one or more ambidentate ligands is bonded to a metal ion.

Example 1. [Pt(NH₃)₃(NCS)]⁺ and [Pt(NH₃)₃(SCN)]⁺

The two isomers differ in the mode of thiocyanate bonding. In the first isomer, thiocyanate is bonded through nitrogen; in the second it is bonded through sulfur.

Example 2. A compound with the formula CoCl₂(NO₂).5NH₃ has two isomers, one yellow and one red. Each precipitates two moles of silver chloride, therefore both chloride ions are outside the cobalt(III) coordination sphere. Neither has an aqueous solution that is basic to pH paper, therefore all the ammonias are bonded to cobalt (III).

The obvious possibility is that the ambidentate nitrite group is differently bonded in these two complexes: [Co(NH₃)₅NO₂]Cl₂ and [Co(NH₃)₅ONO]Cl₂

Werner assigned structures to each based on color similarities to other known cobalt(III) complexes. [Co(NH₃)₆]Cl₃ is yellow and has 6 nitrogen donor atoms bonded to cobalt; [Co(NH₃)₅(OH₂)]Cl₃ is red and has 5 nitrogen donor atoms and an oxygen bonded to cobalt. Thus the yellow isomer of CoCl₂(NO₂).5NH₃ contains N-bonded nitrite and is [Co(NH₃)₅NO₂]Cl₂; the red isomer contains O-bonded nitrite and is [Co(NH₃)₅ONO]Cl₂.

Today we would assign the structures on the basis of infrared spectra: N- and O-bonded nitrite have different N-O stretching frequencies.

3. Coordination isomers involve ligand exchange between coordination spheres of two metal ions that are part of the same compound.

Example 1. Two compounds are known that contain two Pt(II) ions, four ammonia molecules, and four chloride ions. They are:

[Pt(NH₃)₄]²⁺[PtCl₄]^2- and [Pt(NH₃)₃Cl]⁺[Pt(NH₃)Cl₃]^-

[Pt(NH₃)₂Cl₂] has the same ratio of atoms, but does not have the same overall formula; hence it is not a coordination isomer of the above compounds.

Example 2. Three compounds are known that contain one Pt(II) ion, one Pt(IV) ion, four ammonia molecules, and six chloride ions. They are:

[Pt^II(NH₃)₄]²⁺[Pt^IVCl₆]^2-
[Pt^II(NH₃)₃Cl]⁺[Pt^IV(NH₃)Cl₅]^-
[Pt^IV(NH₃)₄Cl₂]²⁺ [Pt^IICl₄]^2-

TEST YOURSELF: Coordination Isomers. Are there other coordination isomers besides these three that contain one Pt(II) ion, one Pt(IV) ion, four ammonia molecules, and six chloride ions? If so give their condensed strucutres as has been done above. Answer

C. Stereoisomers in Metal Complexes: an Introduction

1. Geometric Isomers are found in square planar and octahedral complexes. Examples for square planar coordination are the cis- and trans-isomers of diamminedichloroplatinum(II):

Note the convention of drawing a square with the metal ion in the center and the ligands at the corners of the square.

An example of geometric isomers in octahedral complexes are the cis- and trans-isomers of the tetraamminedichlorocobalt(III) ion:

Note that there are several different ways to represent an octahedrally coordinated metal ion; which way you choose depends on what you are trying to show.

2. Optical Isomers are usually associated with tetrahedral or octahedral geometries. The [Co(NH₃)ClBrI]^- ion is tetrahedral, with four different groups bonded to the cobalt, and has two nonsuperimposable mirror images:

Thus this complex is exactly analogous to carbon compounds. It is also true that an octahedral metal ion bonded to 6 different ligands would be chiral (there are no known examples of such a complex as far as I know):

However, there are instances of chirality and optical isomerism that do not depend on having four different groups attached to a tetrahedral central atom. For example, thetris(ethylenediamine)cobalt(III) ion is chiral, in spite of the fact that the three ethylenediamine ligands are all the same and are themselves symmetrical:

One of the alternate ways of showing the octahedron shows the right/left handed nature of these two isomers more clearly:

Additional examples of geometric and optical isomerism will be considered in the next lecture.

SUMMARY SELF TEST: Sketch all of the isomers possible (including geometric) for a compound with the formula Pt(NCS)₂SO₄.4NH₃. Platinum(IV) is 6-coordinate with an octahedral geometry. Assume that the ammonia molecules are bonded to the platinum(IV) in every isomer. Answer.

This page is http://chemiris.labs.brocku.ca/~chemweb/courses/chem232/CHEM2P32_Lecture_6.html
Created January 18, 2001 by M. F. Richardson
© Brock University, 2001