Isomerism in co-ordination compounds

Compounds having the same molecular formula but different

structures or spatial arrangements are called isomers and
the phenomenon is referred as isomerism.

(1) Structural isomerism : Here the isomers have different

arrangement of ligands around the central metal atom. It is
of the following types :
(i) Ionization isomerism :The co-ordination compound
having the same composition or molecular formula but gives
different ions in solution are called ionization isomers.
There is exchange of anions between the co-ordination sphere and ionization sphere.
E
[COBr(NH3)5]SO4 [Co SO4(NH3)5]Br
Pentaaminebromo Pentaaminesulphato
cobalt (III) Sulphate cobalt (III) bromide
SO42– present in Br– present in ionization
ionisation sphere sphere
Gives white precipitate Gives light yellow
with BaCl2 precipitate with AgNO3
xample :
(ii) Co-ordination is omerism : In this case compound is
made up of cation and anion and the isomerism arises due to
interchange of ligands between complex cation and complex
anion.
Example: [Co(NH3)6][Cr(CN)6]

[Cr(NH3)6][Co(CN)6]
hexaamine cobalt (III) hexacyano chromate (III) hexaamine
chromium (III) hexacyanocobalt (III)

complex cation contains → NH3 ligand (with cobalt) complex

anion contains → NH3 ligand (with chromium)

complex anion contains → CN– ligand (with chromium)

complex anion contains → CN– ligand (with cobalt)

(iii) Linkage isomerism :In this case isomers differ in the

mode of attachment of ligand to central metal ion and the
phenomenon is called linkage isomerism.
Example: [Co ONO(NH3)5]Cl2; [Co NO2(NH3)5]Cl2

Pentaamminenitritocobalt (III)chloride
Pentaaminenitrocobalt (III) chloride
:O – NO– oxygen atom donates lone pair of electrons (nitrito)
NO2– nitrogen atom donates lone pair of electrons (nitro)

(iv) Hydrate isomerism :Hydrate isomers have the same

composition but differ in the number of water molecules
present as ligands and the phenomenon is called hydrate
isomerism.

Examples :(a) [Cr(H2O)6]Cl3 hexaaquachromium (III) chloride

(violet)
(b) [Cr(H2O)5Cl]Cl2.H2O pentaaquachlorochromate (III)
chloride monohydrate (blue green)
(c) [Cr(H2O)4(Cl)2]Cl.2H2O tetraaquadichloro chromiumate (III)
chloride dihydrate (green)
(2) Stereo isomerism or space isomerism : Here the
isomers differ only in the spatial arrangement of atoms of
groups about the central metal atom. It is of two types :
(i) Geometrical or Cis-trans isomerism : This isomerism
arises due to the difference in geometrical arrangement of
the ligands around the central atom. When identical ligands
occupy positions near to each other called cis-isomer.
When identical ligands occupy positions opposite to each
other calledtrans –isomer. It is very common in
disubstituted complexes with co-ordination number of 4 and
6.
• Complexes of co-ordination number 4

Tetrahedral geometry : In this case all the four ligands are

symmetrically arranged with respect to one another as such
geometrical isomerism is not possible.
Square planar geometry : The four ligands occupy
position at the four corners and the metal atom or ion is at
the center and lie in the same plane.
Type: [Ma2b2], M = Pt, a = Cl, b = NH3
Example: [PtCl (NH3)(Py)2]

Complexes of co-ordination number 6 Octahedral

geometry : Here the metal atom or ion lies at the center
and 1 to 6 position are occupied by the ligands.
Cis–Positions : 1–2, 2–3, 3–4, 4–5
Trans – position : 1–4, 2–5, 3–6
Type-I: Ma4b2, M = Co, a = NH3, and b = Cl
Example: [CoCl2 (NH3)4]+ ion
Type –II [Ma3b3], M = Rh, a = Cl and b = Py
Example : [RhCl3(Py)3]
Type –III [M(aa)2(en)2]++, M = Co, a – a = CH2NH2
|
CH2NH2
(bidentate)

b = Cl (monodentate)

(ii) Optical isomerism

(a)Optical isomers are mirror images of each other and have
chiral centers.
(b) Mirror images are not super imposable and do and have
the plane of symmetry.
(c) Optical isomers have similar physical and chemical
properties but differ in rotating the plane of plane polarized
light.
(d) Isomer which rotates the plane polarized light to the right
is called dextro rotatory (d-form) and the isomer which
rotates the plane polarized light to the left is called
laevorotatory (l–form)
Isomers are compounds that have the same molecular formulas but differ slightly in
structure or composition. Structural isomers only differ in structure or bond type. There
are three types of structural isomers: ionization, coordination and linkage.

Introduction
Structural isomers, as their name implies, differ in their structure or bond type. This is
different from stereoisomers, which differ in where the ligands are attached but still have
the same kindsof ligands attached. The notable difference between structural isomers and
stereoisomers is structural isomers have basically the same chemical formulas but with
different bonding arrangements, while stereoisomers have identical chemical formulas.
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

We have pentaaquabromocobaltate(II)chloride which changes to

pentaaquachlorocobaltate(II)bromide.

Coordination Isomerism
Coordination isomers occur with coordination compounds that are composed of both a
cation complex and an anion complex, meaning 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
Linkage Isomerism
The linkage isomers of a coordination complex have the same ligands and central atom,
and the ligands are attached in the same locations. 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 (NO 2-) 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. 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.
Example

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.

Answers
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.

Stereoisomers
Stereoisomers are isomers that have the same molecular formula and ligands, but
differ in the arrangement of the ligands in 3D space.
Introduction
Before we jump to stereoisomers, let us quickly review what isomers are. Isomers are
molecules that have the same molecular formula but differ in the way the atoms are
arranged around the central atom. For example, a molecule with the formula AB 2C2, has
two ways it can be drawn:
Isomer 1: Isomer 2:

The above two pictures are examples of isomers, specifically cis-trans isomers which we
will discuss later on. One thing to remember whenever talking about stereoisomers is that
all other options that may come to mind are just rotations of the existing isomers. For
example, a molecule that has both C's on the top right plane and both B's on the bottom
left plane is not another isomer. Instead it is just a 180o rotation of isomer 2.
Stereoisomers are isomers that mainly differ in the way the ligands or atoms are placed
relative to the central atom. There are two types of stereoisomers:
 Geometric Isomers = Isomers that differ in the way the ligand is bound to the
metal.
 Optical Isomers = Isomers that do not have symmetry and are not
superimposable on their mirror images.

In terms of their differences, geometric isomers show much more activity than optical
isomers. What this statement means is that optical isomers often display similar
properties and do not seem all that different. That is until they react with other optical
isomers or when they react with light. As discussed later on, one of the main ways optical
isomers are detected is their ability to polarize or change the direction of light. Geometric
isomers on the other hand exhibit different properties from one isomer to another.

Review of Molecular Geometry

To really understand stereoisomers, one must understand all the possible molecular
geometries. For stereoisomers, only these geometries will be relevant:
 Linear
 Square Planar
 Tetrahedral
 Octahedral
It is also important to remember the coordination number associated with these
geometries. Coordination numbers refer to the number of ligands or atoms bound to the
central atom. Thus, linear has a coordination number of 2 because it consists of 2 atoms
bound to the central atom. Square planar and tetrahedral have a coordination number of 4
while octahedral has a coordination number of 6.
These geometries are very important as they dictate whether or not certain isomers exist.

Linear
An example of linear geometry is provided below of the molecule Xenon Difluoride
(XeF2). Recall that linear is the geometry where the molecule looks like a line.
XeF2:

Square Planar
Square planar is the geometry where the molecule looks like it is a square plane. An
example of the molecule Xenon Tetrafluoride (XeF4) is provided below.
XeF4:

Tetrahedral
Tetrahedral is the geometry where the molecule looks like a pyramid. An example of the
molecule Methane (CH4) is provided below.
CH4:
Octahedral
Octahedral is the geometry where the bases of two pyramids are stuck together.
Alternatively, it can also be though as where the center consists of a square plane with a
ligand sticking out above it and another ligand sticking out below it. An example of the
molecule Sulfur Hexafluoride (SF6) is provided below. A quick note on octahedral
geometry: sometimes an octahedral molecule may contain polydentate ligands.
Polydentate ligands are ligands that "bite" the central atom in several locations. In other
words, polydentate ligands have the ability to form more than one bond with the central
metal, unlike other ligands. An example of a polydentate ligand is ethylenediamine, a
bidentate ligand, which is abbreviated as en.
SF6:

Geometric Isomers
Geometric Isomers are isomers that differ in the arrangement of the ligands around the
metal or the central atom. In other words, these isomers differ from each other based on
where the ligands are placed in the coordinate compound. This will be much easier to
understand as examples will be considered.
There are 2 main types of geometric isomers:
 Cis-Trans Isomers
 Mer-Fac Isomers
Cis-Trans Isomers
Cis-Trans Isomers are isomers that differ in the arrangement of 2 ligands in square planar
and octahedral geometry. Cis isomers are isomers where the two ligands are 90 degrees
apart from one another in relation to the central molecule. This is because Cis isomers
have a bond angle of 90o, between two same atoms. Trans isomers, on the other hand, are
isomers where the two ligands are on opposite sides in a molecule because trans isomers
have a bond angle of 180o, between the two same atoms. When naming cis or trans
isomers, the name begins either with cis or trans, whichever applies, followed by a
hyphen and then the name of a molecule. For example a cis isomer of CoCl 2F2 would be
called cis-CoCl2F2. Finally, the last thing to keep in mind when examining cis and trans
isomers is that only square planar and octahedral geometries can have cis or trans
isomers. Examples of both isomers are provided below.
Tetrahedral Cis Isomers
CoCl2F2:
(Color scheme: pink=cobalt, blue=fluorine, green=chlorine)

This above is an example of the molecule cis-CoCl 2F2 or cis-dichlorodifluorocobalt (IV).

The molecule pictured above is a cis isomer because both fluorine and chlorine ligands,
respectively, are on the same side of the molecule. Additionally, one can approximate
that the bond angle between each of the chlorine atoms and between each of the fluorine
atoms is 90o.
Tetrahedral Trans Isomers
CoCl2F2:
(Color scheme: pink=cobalt, blue=fluorine, green=chlorine)
(Note, the differences in the length of the bond between the two pictures are not
intentional and have nothing to do with cis-trans isomerism)
This above is an example of the molecule trans-CoCl2F2 or trans-dichlorodifluorocobalt
(IV).
We know the above molecule is a trans isomer because the two same chlorine atoms and
the two same fluorine atoms are opposite each other. Furthermore, the bond angle
between the two chlorine atoms and between the two fluorine atoms is 180 o. The above
examples were all for square planar geometry but as the examples below illustrate, cis-
trans isomerism can also occur in octahedral geometry. Both the molecules below are
isomers of the molecule SCl2F4 (color scheme: yellow=sulfur, blue=fluorine,
green=chlorine).
Octahedral cis Isomers
SCl2F4:

We know this isomer above is a cis isomer because both the chlorine ligands are on the
same side and the bond angle between the chlorine atoms appears to be 90o.
Octahedral Trans Isomers
SCl2F4:
We know this isomer above is a trans isomer because the chlorine ligands are on opposite
sides and the bond angle between the chlorine atoms is 180 o. All other isomers are
essentially just rotations of these two isomers. Once again when trying to find cis and
trans isomers look at the arrangement of the ligands. If two same ligands are on the same
side, it is a cis isomer and if the ligands are on opposite sides, it is a trans isomer. Another
way to tell the isomers apart is the bond angles: cis isomers have a 90 o bond angle
whereas trans isomers have a 180o bond angle.

Mer-Fac Isomers
Mer-Fac isomers are easier to notice than cis-trans isomers in the sense that they only
exist in octahedral geometry. Just like cis-trans isomers, mer-fac isomers are determined
based on whether or not the ligands exist on the same side. Instead of dealing with 2
ligands, mer-fac isomers deal with 3 ligands. If the 3 ligands are all on the same side, the
isomer is called a fac-isomer. Another way to identify fac isomers is to look at the bond
angle between the ligands because fac isomers have a 90 o bond angle between each of the
3 atoms. The mer isomer on the other hand is where only 2 of the 3 ligands are on the
same side. In mer isomers, there exists a 90 o-90o-180o bond angle between the 3 same
ligands. In terms of nomenclature, mer-fac isomers follow the same rule as cis-trans
isomers where you put the isomer type, followed by a hyphen, followed by the molecular
formula. Examples have been provided below.
Fac Isomers Example
Below is an example of the fac isomer, fac-CoCl3F3:
(note the color scheme: pink=cobalt, green=chlorine, blue=fluorine)
2d version:

Through the 2d version, it is easier to see how the ligands are all on the same side.
Nonetheless, in the 3D version, one can observe that the bond angle between the 3 same
ligands is 90o, thus making this isomer a fac-isomer.
Mer Isomers Example

Below is an example of the mer isomer, mer-CoCl3F3:

(note the color scheme: pink=cobalt, green=chlorine, blue=fluorine)

2d version:

Its hard to tell in the 3D version but in the 2D version, one can easily tell how the
same ligands are not on the same side. Additionally, one can approximate that the
bond angle between the three chlorine atoms and between the three fluorine atoms is
90o-90o-180o, thus making the above molecule a mer isomer.

Optical Isomers (Enantiomers)

Optical isomers are isomers in octahedral and tetrahedral geometry that do not exhibit
symmetry and do not have identical mirror images. Optical isomers are difficult to
understand because one must be able to visualize them in a 3D manner.
Before we jump into identifying optical isomers, let's learn some of the terminology
associated with optical isomers. Optical activity refers to whether or not a coordinate
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.
Let's go through a quick review of symmetry and mirror images. A mirror image of an
object is that object flipped or the way the object would look in front of a mirror. For
example, the mirror image of your left hand would be your right hand. Symmetry on the
other hand refers to when an object looks exactly the same when sliced in a certain
direction with a plane. For example imagine the shape of a square. No matter in what
direction it is sliced, the two resulting images will be the same.
As we will discuss later, optical isomers have the unique property of rotating light. When
light is shot through a polarimeter, optical isomers can rotate the light so it comes out in a
different direction on the other end. A youtube video has been attached below in the
outside links section that further explains how to discern optical isomers and their ability
to change the direction of li
Armed with the knowledge of symmetry and mirror images, optical isomers should not
be very difficult. There are two ways optical isomers can be determined: using mirror
images or using planes of symmetry.

Plane of Symmetry Method

The plane of symmetry method uses symmetry, as it's name indicates, to identify optical
isomers. In this method, one tries to see if such a plane exists which when cut through the
coordinate compound produces two exact images. In other words, one looks for the
existence of a plane of symmetry within the coordinate compound. If a plane of
symmetry exists, then no optical isomers exist. On the other hand, if there is no plane of
symmetry, the coordinate compound has optical isomers. Furthermore, if a plane of
symmetry exists around the central atom, then that molecule is called achiral but if a
plane of symmetry does not exist around the central molecule, then that molecule has
chiral center.

Mirror Images Method

The mirror images method uses a mirror image of the molecule to determined whether
optical isomers exist or not. If the mirror image can be rotated in such a way that it looks
identical to the original molecule, then the molecule is said to be superimposable and has
no optical isomers. On the other hand, if the mirror image cannot be rotated in any way
such that it looks identical to the original molecule, then the molecule is said to be non-
superimposable and the molecule has optical isomers. Once again, if the mirror image is
superimposable, then no optical isomers but if the mirror image is non-superimposable,
then optical isomers exist.
Non-superimposable means the structure cannot be rotated in a way that one can be put
on top of another. This means that no matter how the structure is rotated, it cannot be put
on top of another with all points matching. An example of this is your hands. Both left
and right hands are identical, but they cannot be put on top of each other with all points
matching.
Example
Consider the tetrahedral molecule, CHBrClF (note the color scheme: grey=carbon,
white=hydrogen, green=chlorine, blue=fluorine, red=bromine)
Is this molecule optically active? In other words, does this molecule have optical
isomers?
First take the Mirror-image method. The mirror image of the molecule is:

Note that this mirror image is not superimposable. In other words, the mirror image
above cannot be rotated in any such way that it looks identical to the original molecule.
Remember, if the mirror image is not superimposable, then optical isomers exist. Thus
we know that this molecule has optical isomers.
Let's try approaching this problem using the symmetry method. If we take the original
molecule and draw an axis or plane of symmetry down the middle, this is what we get:
Since the left side is not identical to the right, this molecule does not have a symmetrical
center and thus can be called chiral. Additionally, because it does not have a symmetrical
center, we can conclude that this molecule has optical isomers. In general, when dealing
with a tetrahedral molecule that has 4 different ligands, optical isomers will exist most of
the time.
No matter which method you use, the answer will end up being the same.
Example
This time we will be analyzing the octahedral compound FeCl3F3 (note the color scheme:
orange=iron, blue=fluorine, green=chlorine):

If we try to attempt this problem using the mirror image method, we notice that the
mirror image is essentially identical to the original molecule. In other words, the mirror
image can be placed on top of the original molecule and is thus superimposable. Since the
mirror image is superimposable, this molecule does not have any optical isomers. Let's
attempt this same problem using the symmetry method. If we draw an axis or plane of
symmetry, this is what we get:

Since the left side is identical to the right side, this molecule has a symmetrical center and
is an achiral molecule. Thus, it has no optical isomers.

What is a polarimeter?
A polarimeter is a cylinder tube which a single ray of light can be shot through. Another
way to distinguish whether a molecule is chiral or achiral is to shine light through that
molecule. If no light passes that molecule then the molecule is achiral and does not have
optical isomers. On the other hand, if light passes through the molecule, it is chiral and
has optical isomers. Another interesting fact about optical isomers is that they have the
ability to polarize and rotate light passed through them. These type of optical isomers are
classified based on how they rotate the light. If the isomer rotates the light in the left
direction, then it is called levorotatory. If the isomer rotates the light in the right
direction, then it is called dextrorotatory. Often times, certain drugs or proteins also
depend on stereoisomers for function because only one correct stereoisomer of the
molecule can function effectively as the drug or protein.

Applications
Stereoisomers have a lot of applications in biology as well as our day to day lives.
Surprisingly, our own tongue contains chiral molecules that help us discern the taste of
some of the foods we eat. For example, we may eat two of the same leaves but one may
taste bitter and the other may taste sweet because of chirality. As discussed above, optical
isomers also have the ability to rotate light in certain directions. In biological terms,
chirality is key to the proper functioning of an enzyme. This is because chirality allows
the enzyme to function efficiently by being able to bind to only certain substrates.
Problems

1) Draw all possible geometric isomers for the tetrahedral molecule MnCl2F2.

2) What type of isomer is the molecule pictured below? Are there any other types of
isomers possible for this molecule?

3) Draw all possible stereoisomers for the molecule CrF3I3.

4) What is the difference between a dextrorotatory optical isomer and a levorotatory

optical isomer?

5) Draw the molecule Fe(en)3 and state if it is optically active or not.

6) True or False: An octahedral molecule can have cis-trans isomers?

Answers
1) MnCl2F2 has no geometric isomers because recall that tetrahedral molecules do not
have geometric isomers.
2) The molecule, FeBr2I2 pictured in problem 3 is a cis isomer, or cis-FeBr 2I2, because
both the Br and I ligands are on the same side. There is one more stereoisomer for this
molecule: trans-FeBr2I2, pictured below:
3) This molecule has two isomers: mer-CrF3I3 and fac-CrF3I3, both are pictured below.
The mer isomer is where the ligands are not on the same plane and there exists a 90-90-
180 degree bond angle between the 3 same ligands. The fac isomer is where the ligands
are on the same plane and there exists a 90-90-90 degree bond angle between the 3 same
ligands.

fac-CrF3I3:
mer-CrF3I3:

4) A dextrorotatory optical isomer is a isomer that can rotate light in the right direction. A
levorotatory optical isomer on the other hand is a isomer that can rotate light in the left
direction.
5) In the molecule Fe(en)3, recall that the en ligand, ethylenediamine is a bidentate
ligand. The 2D structure of this molecule is shown below:

Note, the black lines represent the bonds and the blue lines represent the bonds
ethylenediamine binds to. Using either the symmetry method or the mirror image method,
one can observe that this molecule has a chiral center and that the mirror image is not
superimposable on the original molecule. Thus, this molecule is optically active because
it has optical isomers.
6) True. Octahedral geometry as well as square planar geometry can have cis-trans
isomers. The only geometry that cannot have cis-trans isomers is tetrahedral.
Ionization