Dr.
Sajjad [CHEM-305]
Isomerism
Hussain Sumrra Inorganic Chemistry-II
Isomerism in
Coordination Compounds
Dr. Sajjad Hussain Sumrra
Key Points
Isomerism in Coordination Compounds
Types of Isomerism
Geometrical Isomerism
Optical Isomerism
Structural Isomerism
Conformational Isomerism
3
Isomerism
Two or more compounds with the same formula
but different arrangements of the atoms are called
isomers
The difference in structure is usually maintained in
solution.
There are many different classes of isomers, like
stereoisomers, enantiomers, and geometrical
isomers.
4
Isomerism in in Coordination Compounds
The existence of coordination compounds with the
same formula but different arrangements of the ligands
was crucial in the development of coordination
chemistry.
Metal complexes exhibit several different types of
isomerism; the two most important are geometrical
and optical.
Note that, in general, only complexes which react
slowly are found to exhibit isomerism.
Complexes that react rapidly often rearrange to yield
only the most stable isomer.
5
Types of Isomerism
There are two main forms of isomerism: structural
isomerism and stereoisomerism (spatial isomerism).
6
Geometrical Isomerism
• In metal complexes, ligands may occupy different positions
around the central atom.
• Since the ligands in question are usually either next to one
another (cis) or opposite each other (trans), this type of
isomerism is often also referred to as cis–trans isomerism
or geometric isomerism.
• Such isomerism is not possible for complexes with
coordination numbers of 2 or 3 or for tetrahedral
complexes. In these systems, all coordination positions are
adjacent to one another.
• However, cis–trans isomerism is very common for square
planar and octahedral complexes.
7
Platinum(II) complexes are very stable and slow to react;
among them are numerous examples of square planar
geometrical isomers. The best known of these are cis- and
trans-[PtCl2(NH3)2].
The cis-isomer has been extensively studied, since this
molecule has had notable success as an anticancer drug.
It is now marketed as cis-Platine, being an almost certain
cure for testicular cancer. 8
The chemistry of platinum(II) complexes has been
examined in detail, particularly by Russian chemists.
Many compounds of the types cis- and trans-[PtX2A2],
[PtX2AB], and [PtXYA2] are known.
• (A and B are neutral ligands such as NH3, py, P(CH3)3,
and S(CH3)2.
• X and Y are anionic ligands such as Cl–, Br–, I–, NO2–,
and SCN–).
Isomers can be distinguished by X-ray diffraction and
a variety of other techniques.
A few compounds of platinum(II) containing four
different ligands, [PtABCD], are known.
9
Realizing that either B, C, or D groups may be trans to
A, it is apparent that there are three isomeric forms for
such a compound.
Similarly in [PtNO2NH3(NH2OH)py]+.
10
Geometrical isomerism is also found in square planar
systems containing unsymmetrical bidentate ligands,
[M(AB)2].
Glycinate ion, NH2CH2COO–, is such a ligand; it
coordinates with platinum(II) to form cis- and trans-
[Pt(gly)2] having structures.
It is not necessary for the attached ligand atoms to
differ; all that is required is that the two halves of the
chelate ring be different
11
Geometrical Isomerism in Octahedral Complexes
Geometrical isomerism in octahedral compounds is very
closely related to that in square planar complexes.
Among the most familiar examples of octahedral
geometrical isomers are the violet (cis) and green (trans)
forms of the tetraamminedichlorocobalt(III) and
chromium(III) cations.
12
The largest number of geometrical isomers would exist
for a complex of the type [MABCDEF], wherein each
ligand is different.
Such a species can exist in fifteen different geometrical
forms (each form would also have an optical isomer).
The only compound of this type that has been prepared
is [Pt(NO2)(Cl)(Br)(I)(py)(NH3)].
It was originally obtained in three different forms and
no attempt was made to isolate all fifteen isomers.
13
Unsymmetrical bidentate ligands give rise to
geometrical isomers as for square planar complexes.
For example, the cis–trans isomers of
triglycinatochromium(III) have the structures.
In [MA3B3] type complexes, the cis-isomer is called
facial, since the like donor atoms are on the same face
of the octahedron. The trans-isomer is called
meridional.
14
Facial and Meridional Isomers
When three identical ligands occupy one face of an
octahedron, the isomer is said to be facial, or fac.
In a fac isomer, any two identical ligands are adjacent
or cis to each other.
If these three ligands and the metal ion are in one plane,
the isomer is said to be meridional, or mer.
A mer isomer can be considered as a combination of a
trans and a cis, since it contains both trans and cis pairs
of identical ligands.
15
Examples of fac and mer isomers.
16
Optical Isomerism
Optical isomerism has been recognized for many years.
The classical experiments in 1848 of Louis Pasteur, one
of the most illustrious and humane of all men of science,
showed that sodium ammonium tartrate exists in two
different forms.
Crystals of the two forms differ, and Pasteur was able to
separate them by the laborious task of hand picking.
Aqueous solutions of two isomers had the property of
rotating a plane of polarized light (a beam of light
vibrating in only one plane) either to right or to the left.
Because of this property the isomers are said to be
optically active and are called optical isomers.
17
Optical Isomerism
18
One is designated as the dextro (+) isomer and the
other as the levo (–) isomer.
The extent of rotation of the plane of polarized light by
the two isomers is exactly the same; however, the
dextro isomer rotates the plane of light to the right, the
levo isomer to the left.
It follows that the rotations cancel each other in
solutions containing equal concentrations of the two
isomers.
Such a (+), (–) mixture is called a racemic mixture.
Since in solution it does not rotate a plane of polarized
light, it is optically inactive.
19
The geometric relationship of most optical isomers is
similar to that of the right and left hands, or feet, or
gloves, or shoes.
There is a rather subtle difference between the structures;
the relative positions of the thumb and fingers on each
hand are the same, yet the two hands are different.
One is the mirror image of the other.
20
In order for a molecule or ion to be optically active, it
must not have a plane of symmetry; i.e., it should not
be possible to divide the particle into two identical
halves.
The most basic test that can be applied in attempting to
decide whether a given structure will be optically active
is to compare it with its mirror image.
If the structure and its mirror image are different, the
structure is said to be dissymetric and the substance will
normally exhibit optical activity.
The (+) and (–) isomers of a given compound are called
enantiomers, which mean “opposite forms”.
They have almost identical chemical and physical
properties.
21
They differ primarily in the direction in which they rotate
a plane of polarized light.
This property permits them to be readily detected and to
be distinguished. A simple instrument known as a
polarimeter is used for this purpose.
Note that the physiological effects of enantiomers are
sometimes profoundly different. Thus the (–)-nicotine
that occurs naturally in tobacco is much more toxic than
the (+)-nicotine that is made in the laboratory.
Specific effects such as these are attributed to dissymetric
reaction sites in biological systems. Since enantiomers are
so similar, and since in chemical reactions the two forms
are produced in equal amounts, special techniques are
required to separate the two. This separation process is
called resolution. 22
Often, a single optical isomer will rearrange to give a
racemic mixture; the process is called racemization.
A simple example of a dissymetric molecule is one with a
tetrahedral structure wherein the central atom is
surrounded by four different atoms or groups.
The structures of optical isomers may be represented by
the amino acids.
23
• Tetrahedral metal complexes are generally very
reactive, which makes it extremely difficult to isolate
them in isomeric forms.
• However, complexes containing two unsymmetrical
bidentate ligands can be resolved into optically-active
forms.
• The enantiomers of bis(benzoylacetonato)beryllium(II).
24
• Note that four different groups around the central atom
are not required for optical activity.
• The only requirement is that the molecule and its mirror
image be different.
• Square planar complexes are very seldom optically
active. In most cases (for example, complexes of the type
[MABCD]), the plane of the molecule is a plane of
symmetry.
• Unlike four-coordinated systems, six-coordinated
complexes afford many examples of optical isomerism.
• These are very common among compounds or ions of
the type [M(AA)3].
• For example, the optical isomers of
trioxalatochromate(III) are complexes
25
Trioxalatochromate(III) complex
26
• It was once claimed that optically-active compounds
must contain carbon, but now at least three optically-
active, purely inorganic complexes are known.
• One complex, prepared by Werner to show that carbon
was not required, used the bridged complex, in which
the dihydroxo complex is a bidentate ligand.
• The fact that complexes of the type [M(AA)3] can be
resolved into optical isomers is good evidence that
these complexes have the octahedral configuration.
27
• Another very common type of optically-active complex
has the general formula [M(AA)2X2].
• In this system, it is important to note that the trans
isomer has a plane of symmetry and cannot be optically
active.
• Therefore, the cis structure for such a complex is
conclusively demonstrated if the complex is shown to be
optically active.
• This technique for proof of structure has often been used;
the identity of the cis and trans isomers of the complex
dichlorobis(ethylenediamine)rhodium(III).
28
29
• Multidentate ligands can also give rise to optical
isomerism in metal complexes. One of the many such
cases is that of (+)- and (–)-[Co(EDTA)]–.
30
• A complex containing six different ligands is
dissymetric; each of its fifteen geometrical isomers
should be resolvable into isomers.
•
• Example:[Pt(NO2)(Cl)(Br)(I)(py)(NH3)]
31
In conclusion, note that the designation of an optical
isomer as either dextro or levo is meaningful only if the
wavelength of the light used is known.
An optical isomer may rotate the plane of polarized light
to the right (dextro) at one wavelength but to the left at
another.
The mirror image isomer gives the mirror image curve.
Such plots of optical rotation versus wavelength of light
are called rotatory dispersion curves.
The absolute configuration of (+)-[Co(en)3]3+ was
determined by means of X-ray diffraction studies.
Using this as a standard it has been possible to assign
absolute structures to other complexes by comparison of
their rotatory dispersion curves.
32
The rotatory dispersion curves and the structures for
the optical isomers of [Co(en)3]3+.
33
34
Other Types of Isomerism
Several types of isomerism, other than geometrical and
optical, are known for coordination compounds.
These are often unique to this class of compound.
Structural Isomers
• 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.
• 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. 35
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.
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.
36
Some examples of [MAn][M’Xm] and [M’Am][MXn] are;
[Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4
37
A special type of coordination isomerism is one that
involves the different placement of ligands in a bridged
complex.
This is sometimes called coordination position
isomerism.
38
Some more examples;
Ionization Isomerism:
Ionization isomerism is the term used to describe isomers
that yield different ions in solution. A classical example is
the purple [CoBr(NH3)5]SO4 and red [CoSO4(NH3)5]Br,
which give sulfate and bromide ions, respectively, in
solution.
39
Two sets of the many isomers of this type are;
• [Co(NCS)2(en)2]Cl and [Co(NCS)Cl(en)2]NCS
• [PtBr(NH3)3]NO2 and [PtNO2(NH3)3]Br.
• Very similar to these are the isomers resulting from
replacement of a coordinated group by water of
hydration. This type of isomerism is sometimes called
hydration isomerism.
40
• The best known example is the trio of compounds
[Cr(H2O)6]Cl3, [CrCl(H2O)5]Cl2·H2O, and
[CrCl2(H2O)4]Cl·2H2O, which contain six, five, and
four coordinate water molecules, respectively.
• These isomers differ markedly in physical and chemical
properties.
• Other isomers of the same type are
[CoCl(en)2(H2O)]Cl2 and [CoCl2(en)2]Cl·H2O
[CrCl2(py)2(H2O)2]Cl and [CrCl3(py)2(H2O)]·H2O
Linkage Isomerism:
Isomerism of the linkage type may result whenever a ligand
has two different atoms available for coordination.
41
• It has long been known that nitrite ion in cobalt(III)
complexes can be attached either through the nitrogen,
Co—NO2 (nitro), or the oxygen, Co—ONO (nitrito).
• The nitrito complexes of cobalt(III) are unstable and
rearrange to form the more stable nitroisomers.
• Similar linkage isomers can be obtained in complexes
of rhodium(III), iridium(III), and platinum(IV).
42
Some examples of this type of isomerism are;
• [(NH3)5Co—NO2]C12 and [(NH3)5Co—ONO]Cl2
• [(NH3)2(py)2Co(—NO2)2]NO3 and (NH3)2(py)2Co(—
ONO)2]NO3
• [(NH3)5Ir—NO2]Cl2 and [(NH3)5Ir—ONO]Cl2
43
[(NH3)5Co—NO2]C12 and [(NH3)5Co—ONO]Cl2
[Co(NH3)5(NO2)]Cl2 [Co(NH3)5(ONO)]Cl2
Pentaamminenitrocobalt(III) Pentaamminenitritocobalt(III)
chloride chloride
44
• Many other ligands are potentially capable of forming
linkage isomers.
• Theoretically, all that is required is that two different
atoms of the ligand contain an unshared electron pair.
• Thus the thiocyanate ion, can attach itself to the metal
either through the nitrogen, M—NCS, or the sulfur, M—
SCN.
45
• Each type of attachment does occur, but generally to give
only one form or the other in any particular system.
• Usually the first-row transition elements are attached
through nitrogen, whereas the second- and third-row
transition elements (in particular the platinum metals)
are attached through sulfur.
• The following linkage isomers of this type have been
prepared.
[(bipy)Pd(—SCN)2] and [(bipy)Pd(—NCS)2]
[(OC)5Mn—SCN] and [(OC)5Mn—NCS]
[(H2O)5Cr—SCN]2+ and [(H2O)5Cr—NCS]2+
• Linkage isomers of CN– and of S2O32– metal complexes
have also been reported.
46
Conformational Isomerism
• As was stated earlier, four-coordinated complexes have
either a tetrahedral or a square planar structure.
• The geometry observed in a particular system depends
on the metal and also on the ligands. Thus complexes of
beryllium(II) are always tetrahedral, whereas
nickel(II) forms tetrahedral [NiBr4]2– and square
planar [Ni(CN)4]2–.
• For certain ligands the stabilities of the two structures
may not differ greatly; in such cases both forms may be
obtained and these are called conformational isomers.
47
• Examples of such isomers for nickel(II) and cobalt(II)
are
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