2.

12 STEREOISOMERISM
plane of plane polarized light as it passes through the
vibrates in all directions, plane polarized light vibratessolutions
of them. Unlike ordinary light, which
ofligandssare coordinated to the metal aee
sametypes
and numberstereoisomers In other words, only in single plane. Plane polarized light is
Theisomers
the
in which arrangementsare
called ,stereoisomers are
ligandss coordinated to metal cation or
obtained by passing ordinary light through polarizing filter (.e, nicol prism) which
CaF2. When plane polarized light is passed through the is made upof quartz,
differentspatial arrangement of atom. solution of a chiral
cationbut withdifferonlyinthe spatial two types: polarization is rotated either to the right or to the left. If the plane of polarizationcompound the plane of
is rotated to right, the
isomersthat classifiedinto isomer is said to be
Stereoisomerismis dextrorotatory (d or +), if the plane of polarization is rotated to left, the isomer is said
()Geometricalisomerism to be levorotatory (or-). The d- and /- isomers ofa chiral
substance are called enantiomers. The d- and/
(2)Optical isomerism - isomers rotate the plane of polarized light by the equal amount but in
opposite directions. An
(1) Geometricallsomerism mixture of d - and - isomers, calleda racemic mixture produces no net optical rotation equimolar
orientations of the ligands-or more specifically rotations produced by the individual enantiomers is exactly cancelled. because the
relative positions or
Stereoisomers in which the is different are called Igeometrical isomers andithis
phenomenon is to be
The essential condition for a substance to be chiral (or optically active) is that the substance has no
the metal cation isomers can not be inter-conveted wihout breaking of M. plane of symmetry. Ifa substance has a plane of symmetry, then it will be achiral (or optically
donor atoms round inactive).
geometricatisomerism Geometricat in one isomers the ttwo particular ligands are adjacent to The substances having no plane of symmetry (or mirror plane
symmetry) are always
called isomers exists only in pairs, formula. Thus, the isomer in non-superimposable on their mirror images.
bonds. Geometrical the two are in
opposite sides în the structural Geometrical and optical isomerism in complexes which exhibit coordination number 4.
other
each other and in the non-identical) occupy the adjacent positions of eact
Mich two particular (A)) Tetrhedral Complexes: Tetrahcdral complexes do not exhibit geometrical
isomerism whether
isomer in which two particular 1igands occupy opposite positions to all the ligands are same or different
other is called cis-isomer andthe(The latin word cis means next tto, trans means across). cis- and trant relative to cach other, ie., cach ligandbecause
is
all the ligands in this geometry are at adjacent positions
present
cach other is called trans-isomer at 10928 from cach of the other three ligands.
like melting points, dipole moment
isomers are diferent compounds with different properties cis-[P(NH3)2Cl, ]is polar
Tetrahedral complexes of [Ma4)"*, [Mab2 )"* and [Masb]"* type do not show optical
molecule and s isomerism
solubility, colours and chemical properties. For example, because all the possible arrangements
o the tigands round the metal cation are the same.
more soluble in water than trans- (Pt(NH3)2Cl2 ]molecule wnien navewhere
zeto poe
aHowever,
as the trans isomer (Mabcd)" type tetrahedral complexes show optical isomerism. For example,
[Pt (NH;)2Ch] called cis-platin is an effective anti-cancer drug coordinatice
[As (CH3)(C,Hs XSXCGHsCO0)] ion show optical isomers as shown in Fig. 2.15.
physiologically inactive. Geometrical isomerism is most common in complexes having
and 3 do not exhib
number of 4 and 6. The complexes which exhibit coordination numbers 2 2 2
geometrical isomerism.

(2) Optical or mirror Image Isomerism

mimu
Optical isomers or enantiomers s are pairs of molecules or ions which are non- -superimposable o0ccaH,
HC
CH,Co0 CHa
images of cach other. The tern superimposable means that if one structure is laid over tne vu
means thati
same molecule, the positions of all the atoms will match and dthe term non--Superimposable CHs
one structure is laid over the other of the same notmatch.Fo
molecule, the epositions ofall the atoms will Th
example, ifapipette is placed in front ofa mirror, the image reflected is identical tothepipetteitself b
Mirror
Geometrical isomers 1, Optical isomers 2, Stereoisomers 2
pipette and its miror image both are of a miror,
If the left hand is placed in front Fig. 2.15 Optical isomers of [As*(CH3)XCHs)\S)(CoHscOo).nlj
image reflected will look like the right superimposable.
hand. Thus, we can say that the left hand anddright
r
hand are mi The complex [As (CH; XC,H)\SXC&H;coo) has no plane of symmetry hence is optically active.
image of cach other. However, they are isplacedoverr
because when left hand i andrt Tetrahedral complexes of Be (II), B() and Zn () with unsymmetric chelating ligand also exist as
hand keeping both palms
hands is called handedness.down,
non-superimposable
they do not match. The non- -superimposable propertyVofleft optical
Fig. 2.16.isomers. For example, optical isomers of bis (benzoytacetonato) beryllium are shown in
The chiral (prono
ky-ral, from the greek word, cheir, optical
meaningisomers have like lett t and
hand) because are said
handedness and right
tochiral molecules
hands,
non-superimposablbee. Isomers thator are superimposable (i.e.,
images are said to that lack handedness)
withtheir
theyrose
non-chiral
 CH Py
CH
CH
cis-isomer
CH CH, trans-isomer
Fig. 2.18 cis- and trans- isomers of [Pt (pyh(NH, )C]
CH.
CH, (iv) (Mabed|"* type Complexes :
Mirror
2, Stereoisomers 2 Square planar complexes 0of this type exist in three isomeric foms: (Fig. 2.19)
Geometrical isomers 1,Optical isomers 0

Fig. 2.16 Optical isomers of bis (benzoylacetonato) berylium(ll). HNK Py

(B) Square Planar Complexes: Br
Square planar complexes rarely show optical isomerism whether all the four ligands are different o
same because they have all thefour ligands and the metal cation in the same planeand hence have a plane Fig. 2.19 Three geometrical isomers of [Pt (py) (NH,) CI Br].
f symmetry. However, there are exceptionally some complexes which exhibitoptical isomerism. For
example, isobutylenediaminemeso-diphenylethylenediamine palladium (II) or platinum () complexes The three isomers of[Pr (py) (NH, )CI Br), for example, can be obtained by fixing one ligand, say
(also called as isobutylenediamine meso-stilbenediaminepaladium (1) or platinum () complexes) are NH, at one cormer and then placing the other three ligands, one by one trans- to NH, Fig. 2.19. Some
square planarstructuresand are optically active. The optical isomers of Pt (D or Pd (1) square planar other examples of such type of square planar complexes which can exist in three isomeric forms are :
complex are shown in Fig. 2.24 and 2.25.
o (Ma4t,(Mayb,(M(AA)fM(AA) abj and [M(AA)az type square planar [Pr (CzH4) (NH )CI Br]. [Pt (py) (NH, )(NH,OH) (NO, )] etc.
complexes do not exhibit geometrical isomerism because all the possible spatial () IM (AB)2|"* type Complexes :
ligands round the metal cation is the säme arrangement
of the
Here Mis the central metatcarto andAB is an unsymmetrical bidentate ligandin which Aand Bare
() [Mazb;|* type Complexes : two different donor atoms. Examples of such type ofcomplexes are :(Pt (gly)21.[Cu (gly)2Jete.
Thecis
Examples of this type of complexes are : [Pt(NH));Cl; . [Pt(py);Cl2] etc. which and trans- isomers of [Pr(gly)2 Jare shown in Fig. 2.20.
geometrical isomerism. cis- and trans- isomers of[Pt (NH)Cl1, for example, are shown in Fig.exhibit
2.17.
HN q0 OR
HNK

cis-isomer
cis-isomer
Fig. 2.17 cis- and trans- trans-isomer
isomers of [Pt (NHhChl square
() [Mazbe|* type planar complexes.
Examples of this type of Complexes :
CO
OR

[Pt complexes are

[Pt (NH)2(NO2 Ci] which exhibit geometrical [P(NH, )2 pyCij. [Pt (py)2 (NH; )c
(py)2 (NH, CI) , forctc.example, trans-isomer
are shown in Fig 2.18, isomerism. cis and trans- Fig. 2.20 cis- and trans- isomers of [Pt (glyla] where gly =NH, CH, COO.
isomers O (vi) Bridged Binuclear Square Planar Complexes of M2az b4 type :
The bridged binuclear square planar complex of M2agb4 type can exist in three isomeric forms
(CIs-, trans- and unsymmetric). The three isomeric forms of[Pt (PEt) Cl2l2, for example, are shown in
ig 221. But only cis- and trans- isomers of most of the complexes of this type have been found.
 Coordination Chemistry Structure and lsomerism in Coordination Compounds
2.17

forms a square planar complex ion with palladium (I) in

2.16
C
(ix) Ethylenediaminetetraacetate, EDTA ion exists as optical isomers as shown in Fig.
C which EDTA acts as a tetradentate ligand. This complex
PEt q PEt3
Unsymnetric
PEty
2.25
EtgP
binuclear bridged complex. 2

Fig. 2.21 Geometrical

isomers of Ptz (PEtyh Cl
carrying one or more substie
CH,COO
I,CH-cHy
CHCOo oocoha
with symmetric bidentate ligands
(v) Square planar complexes example, in cis - and trans - isomeric
forme :
For [Pt (pn)2] exists plane of
can form geometrical isomers. with respect to the median CO-0 CO
and trans- respectively
2.22) in which the methyl groups are cis-
-0-co
2+
ring atoms.
CHNNH -CH,
j2+ CHHNNH,CH, Mirror
=2ich
Stereoisomers
Geometrical isomers= 1, Optical isomers =2,
CH
CHsHN Fig. 2.25 Stereoisomers of [Pd
ion.
(EDTA)
Complexes
H
Geometrical and optical Isomerism in Octahedral of an octahedron and the six
complexa metal cation will present in the centre
trans-isomer
cisisomer
In an octahedral from 1l to 6 as shown in Fig. 2.26.
Fig. 2.22 cis- and trans- isomers of (Pt (pn)b.
ligands occupy the six corners numbered
substituents, like
An another example of complex is that having bidentate ligands with two methyl
[P(bn)2 ] where bn is 2,3-diaminobutane as shown in Fig. 2.23.
CHJ2+ H u , C H , j2+
H -H,NK NHH

NH, CH, CH,

metal ion Mn*,
in an octahedral complex round the
Fig. 2.26 Arrangement of six ligands ligands of interest occupy the
cissomer or some times two different
trans-isomer In cis - isomers the two similar ligands In cis- isomers the same ligands occupy either of the
Fig. 2.23 cis- and trans- isomers of [Pt (bnb1 corners of octahedral adjacent to one another.
(6,5). In trans- isomers
(5,2), (6,2), (6,3), (6,4) orthrough
(vii) positions (1,2), (1,3), (1,4), (1,5), (2,3), (3,4), (4,5), the centre of the
complex is Isobutylenediaminemeso-diphenylethylenediaminepalladium()
or platinum (1I): This tn
on a straight line which passes yeither of the positions
optically active and exhibits optical isomerism((Fig. 224). these ligands are lying opposite to one another
octabedron. In trans - isomers the two ligands under consideration will
2 (1,6), (2,4) and ((3,5). equivalent, there are no geometrical isomers of
12+
Since all the corners of a regular octahedron are symmetric bidentate ligand.
complexes of the type [Ma6] "*, [Masbj"* and [M (AA)3 ]where AA is a
H Octahedral complexes of the type of [Ma6*, [Ma_bl* are optically inactive and do not show
presence of plane of symmetry (Fig. 2.27). For example,
CH, CH, optical isomerism because of the plane of symmetry and hence are optically|
ions both have
[Co (NH, )6" and [Co (NH3),Ci
inactive.
Mirror
Geometrial isomers 1,
Optical isomers 2,
Where M-Pd(I) or P()
Fig. 2.24 Stereoisomers of square planar
Stereoisomers2osbodoE
palladium (0) or platinum (1)
isobutylenediaminemeso-stilbenediamine
complexee
 2.18
Both these formns are achiral and optically inactive, Therefore, these forms do not show optical
NH 34 isomerism.
Cl

~NH,
NH,
NH Opticaly inactive
H,Ni-NH,
Optically inactive
l and [Co(NHasCi² NH,
Fig. 227 Structures of[Co(NH fac-opticaly inactive mer-optically inactive

type Complexes : An important

example of this type of complexes is Geometrical isomers =2, Optical isomers 0, Stereoisomers 2
0 [Ma b;| 2.28). Both these forms have
[Co(NH3)a,] ion which exists in cis- and trans- isomeric forms (Fig. Fig. 2.30 Stereoisomers of [Co(NH3 hClal
Thus, this type of complexes do not show optical
plane of symmetry and therefore, are optically inactive. (iv) [Ma, b; C2] type complexes: An important example of this type of complexes is
isomerism out of these only one
NH Pt (NH3)2 (py)2Cl2]* ion. This complex ion exist as five geometrical isomers,
and are non-superimposable
H,N isomer exists as two optical isomers which are mirror image of cach other
on each other, as shown in Fig. 2.31
NH. NH,
H,N H,N SNH, 2+
j2+
NH, C
cis- opticaly inactive trans-opticaly inactive
NH,
Geometrical isomers =2, Optical lsomers0, Stereoisomers2
Fig. 2.28 Stereoisomers of [Co(NH l Cal' lon
T
Py
) Ma4be|"* type Complexes : An important cxample of this type of complexes cis-d-isomer cis--isomer (C)
(a) Mirror (b)
[Co(NH1)4(H;0)Cij ion. This complex ion exists in cis- and trans- forms (Fig. 2.29). Both these
forms are achiral. Therefore, these forms do not show optical isomerism.
Enantiomers
NH,
2+
OH, C
124 NH Py
2+
12+
-SNH,
cis-opticaly inactive
Geometrical isomers =2, Optical isomers 0, Stereoisomers
trans-ogicaly
=2
inacthve Ldl NH, Py
(G) (o)
Fig. 2.29 Stereoisomers of [Co(NH a (HO)ci lon
Fig. 2.31 Stereoisomers (geometrical and optical isomers) of [P(NH3 )b(py)Cl,
() (May by| * type complexes : An important active.
(Co (NH,)Cly]. This complex exists in facial (ie., cis-) and example this type of
of Isomers (a) and (b) are the mirror images (i.e.,, enantiomers) of cach other anddoarenotoptically
exist as optical
meriddional (i.e, trans-) complexes2.30).is Isomers (©), (d), (e) and () are achiral and optically inactive and thus these isomers
forms (Fig ISomers
 2.20 (v) [Mabcdef]"* type Complexes :
Complexes: (Py)2 CIBrt ion. This complex [P (py) (NH )(NO, )(CI) (B) ()] is the only complex of this type. The possible number of
(OMazb;cdj** type is [Co(NH3 )2 1somers exist as optical ison
type of complexes
example of this hence is
geometrical isomers of this complex is 15. Each of these 15 geometrical isomers is chiral and
twogeometrncalfour and the number of pairs
An important isomers and, out of these only(py): of
CIBr] are optically active. Therefore, there are 15 pairs of enantiomers (e, total number of optical isomers is 30).
exists as six geometrical
optical isomers for
(Co(NH3)2
are shown in Fig
2.32. The i
The optical isomers of one of the 15 geometrical isomers are shown in Fig. 2.33.
Thus the number of stereoisomers of[Co(NH)2 (y); CIBr]
enantiomes s two, The of csch other are called the enantiomers
NH.
NH. NH, NH,
images
which are mirror NH,
NH.

Py Py
Py
P
P Mirror
cís-d-isomer cis-fisomer
Mirror
Enantiomers
Mirror
Enantiomers
NH. Enantiomers
Fig. 2.33 Optical isomers of one of the geometrical isomers of (Pt(NH3 Xpy) (NO2) (CI)(Br)(0)
(vi) (M(AA)3]"* type Complexes : These complexes do not show geometrical isomersm.Here
Py (AA) is a symmetric bidentate ligand which may be either a neutral or negative ion. The examples of this
NH,
(V type of complexes are :[Co(en)3].[Cr(ox)3] ctc. cach of which is chiral. Such complexes can exist
Opticaly inactive Optically inactive in either of two enantiomeric forms (i.e., d- and l- isomers) or a racemic mixture of the two. It is to be
noted that an octahedral complex containing threce chelate rings are always chiral and optically active.
Therefore, these complexes arce always exist as pairs of enantiomers,Optical isomers of[Co(en)] and
[Cr(ox)3 ions are shown in Fig. 2.34(a) and 2.34(b).
en N

B Br
VI)
Optically inactive Optically inactive
Geometrical Isomers =6, Optical isomers =4, Stereoisomers=8
Sof enantiomers =2
Fig. 2.32 Stereoisomers (geometrical and optical isomers) and pairs Mirror Äsomer
of enentiomers of disomer Mirror isomer disomer
(Co(NHs lapy2CIBr] ion Geometrical lsomers =1,Optical isomers =2, Geometrical lsomers =1,Optical isomers =2,
The geometrical isomers (1) and (I) have no Stereoiscomers2 Stereoisomers =2
Thusoptical isomers for (Co(NH
plane of symmetry and therefore, are optically active. (b)
isomers (l), (IV), (V) and (VI) 3)2(py);CIBr]' are 4and pairs of (a
have plane of symmetries and, enantiomers 2. The geometrical
are
these isomers have no optical isomers. therefore, are optically inactive. Thus Fig. 2.34 Optical isomers of (a) [Co(en)a| and (b) [Co(ox)] ions
 Structure and lsomerism in Coordination Compounds

of complexes NH
2.22 exampie ot this type
type complexes : An important 1somers, cis isomers is chiral
(vi) (M (AA)) ay]" cis- and trans- the other hand
complex ion exists as in Fig 2.35. On
(Co(en); Cl,] ion. This exists as d- and - isomers as shown trans- isomer does not show
optically active. Thus, it optically inactive, therefore, this
trans-isomer is achiral and cis-lisomer trans-somer
cis disomer Mirror
isomerism.
Achiral and optically inactive
Fig. 2.36 Stereoisomers of [Co (en)(NHs )CH lon.
NH, NH,
HN ,N

cis -somer
cis-disomer Mirror

cisdisomer cisIisomer
Mirror

Chiral and optically active

NH, CI +

transsomer
Achiral and optically inactive
tNH
Fig. 2.35 Sterioisomers of [Co(en )Ch]" ion.
Other examples of [M(AA)2 a2 ]* type complexes are [Cof(en)2 (N02)21 NH,
(CrC;04)2 (H,0)2.[lr (C;04);Cl, etc. trans-isomer trans-somer

(ix) (M(AA)) ab]** type Complexes : An important example of this type of complexes Achiral and optically inactive
[Co(en)2 (NH CU² ion. This complex ion exists in cis and trans- forms, cis- isomer is chiral an Geometrical isomers =3, Optical isomers =2, Stereoisomers=4
optically active. Therefore, it can be resolved into d-and I-isomers. The d- andI-isomers are shown i
Fig. 2.36. On the other hand, the trans- isomer is achiral and optically inactive. Therefore, the trans Fig. 2.37 Stereoisomers of [Cofen)(NHh Cl ion.
isomer can not be resolved into d- and -isomers.
() M(AA)ag ba|** type Complexes : Some examples of this type of complexes are
(xÍ) [M (AB)|* type Complexes : An important example of this type of complexes is
[Co (gly)] This complex show geometrical isomerism and exist in fac- and mer- isomeric forms. Both
[Co (en) (NH, ); Cl, l, [Co(C;04 )(NH3)2 (NO2)2].[Cofen)(py)2 (Cl,] etc. These comnlex inns these isomers are chiral and optically active. Therefore, both these isomers (fac- and mer-) can be
in Fig. 2.38.
show geometrical isomerism. The cis- isomers are chiral and optically active.
I-isomers. The trans- isomers are akheretore, these cissoved nto d- and l- 1Somers. The optical isomers of[Co(gly) ]are shown
somes can be resolved into d-and
isc
inactive and
bence do not show optical isomerism. The optical isomers of (Co
(en)(NH, );Cl,] ion are shown in
Fig. 2.37.
 2.24

fac-Isomer
Mirror disomer Mirror I4somer
taodisomner
Geormetrical isomers 1, Optical isomers 2, Stereoisomers 2
Fig. 2.40 Stereoisomers of [M(AAh(AB))* where AA is symmetric bidentate
ligand and AB is an unsymmetric bidentate ligand.
(xiv) [M(AAN(AB)2l* type Complexes : This type of complexes show geometrical isomerism
and all geometrical isomers are chiral and optically active. Therefore, all the geometrical isomers can
exist in d- and - forms as shown in Fig. 2.41.
mer-I-somer
mer-disomer Mirror

Stereoisomers =4
Geometrical isomers =2, Optical isomers 4,
Fig. 2.38 Stereoisomers of (Co(glyal
(rii) M(AA);(BB) type Complexes: An important example of this type of complexes
[Co(en)2 (ox)] ion. This complex is neither cis- nor trans- but it has one optical isomer as shown
Fig. 2.39. cis-d-isomer cis-isomer
Mirror
NAn

nt
ox

Nen
disomer Mirror Hsomer
Geometrical isomers=1, Optical isomers 2, Stereoisomers=2 trans-disomer trans-I-4somer
Fig. 2.39 Stereoisomers of [Co(en )a(ox)] ion. Mirror

(xii) [M(AA)2(AB)|" type Complexes : This type of

isomerism but these are chiral and optically active. complexes does not show geometrid
isomeric forms as shown in Fig. 2.40. Therefore, these complexes can exist in d- and

trans-d-somer Leeb Mirror trans-l-isomer

Geometrical isomers =3, Optical isomers =6, Stereolsomers =6 lgd

Fig. 2.41 Stereoisomers of [M(AAXAB)21* ion.
 Structure and lsomerism in Coordination Compounds 2.27

2.26 aly
isomer
type Complexes : The geometrical
(rv) [M(AB);a;|"* of complexes is (Co (gly);Cl2 ] ion. NH.
type
An example of this
complex are shown in Fig. 2.442. P
gy
Mirror

glyS01
Mirror
N9YAATKial
NH,

Mirror

gly

Mirro NH.

P:

gly
Mirror

0gy
Mirro

py
gly
Mirror
Optically inactve
Optically inactive NH, NH
Geometrical isomers =5, Optical isomers=6, Stereoisomers =8
Fig. 2.42 Stereoisomers of (Co
(gyhCl ion.
(xvi) [M(AB); ab* type Complexes: gly
An example of this type of
this complex ion are shown in Fig.complexes
is (Co(gy)2 (NH3 XpY)) ion.
2.43. The geometrical isomers of Py PY
Optically inactive Optically inactive
Geometrical isomers =6, Optcal isomers =8, Stereoisomers= 10
Fig. 2.43 Stereoisomers of [Co (en) (pn) (NO2)2l* ion.
 Sr
2.28 Ligands like 1, 2
Octahedral Complexes Containingg
Optically Active
(pn) (NO2)2] ion. 2-iaminopropane: complexes are
[C(ETA2nds such as
(xvil) Octahedral Complexes Containing
EDTA: The important
.[Mg(EDTA)].[Co(EDTA)J etc. These
(rvi) of complex is [Co (en)
example of this type xamples of this type of trans- but these are chiral and optically active. Therefore, they exist as d
An important
en’ CH,-CH; oomplexes are neither cis- nor
shown in Fig. 2.45.
dLisomers. The d- and l-isomers of [Co(EDTA)J are
where (1,2-diaminoethane)

NH, NH,
(12-diamincopropane) Co
CO
and pn -’CH-CH-CH, CH, H,C
NH, NH
of the cis- isomers is chiral
This complex ion exists as two cis- and one trans- isomers, cach forms (Fig. 2.44). On the ob
Therefore, the cis-isomers exist in d- and 1- isomeric
optically active. and therefore, should be achiral and optically inactive
hr
hand, the trans-isomer has plane ofsymmetry
it would be optically active.
this isomer has optically active ligand (pn). Thus, CO

cis-disomer Mirror cis-l4somer

Stereoisomers=2
-No, Geometrical isomers =1, Optical isomers =2,
pn Fig. 2.45 Stereoisomers of [Co(EDTA)J ion.
cisdisomer Mirror cisl-somer Complexes (Le., Polynuclear Complexes) : Optical
(xix) Bridged Binuclear Octahedral
complexes containing bridging ligands
isomerism is not limited to mononuclear complexes. Polynuclear
enN N en

For example, the binuclear Co complex ion shown in Fig. 2.46.

can also exist as d- and - isomers.
chiral and optically active and thus exists as d- and -
exist as cis- and trans- isomers. The cis-isomers is isomer is an
is achiral and inactive and thus this
isomers as shown in Fig. 2.47. The trans- isomer
cisdisomer Mirror cisl-somer internally optically compensated i.e., it is meso form.
NO
NO, NH2
(en),Co Co(en)2
NO2
Fig. 2.46 Structure of [Coz(en)a (NH )\(NO2)1* ion.
trans-disomer Mirror
NO,
Geometrical isomers =3, Optical isomers =6, Stereoisomerstranssomer
s6
Fig. 244 Stereoisomers of
(Co(en)XpnX(NO2 2' lon.
 StructuN
2.30 r-isomer, two CI Co Cl
angles are 90 while in the C mer-
en three CCo CI bond lie in one plane
le fac-isomer, the oneis 180. Also we can say that in mer-isomer three CI ligandsbecause
and meridional each set
en 4
anglesare 90" plane. This complex is called
N
\on
bond
bo NH, ligands lie ina perpendicular
meridian ofa sphere (the largest circle that can
be drawn on the
NH, - nd than he regarded as lying on a
ofligar sphere).
surface ofthe
type Complexes :
(b) (M (AB)]" complexes AB is an unsymmetric
bideentate ligand. Well known example of this type
meridional
In this stype of
]and (Cr (gly)3]. where gly =NH, CH2 C00.The facial and
en
cis-lisomer
smnlexes are :(Co(gly)s
shown in Fig. 2.49.
Jare
cis-disomer
Mirror
isomers of[Co (gly) 3
10
en

NH
er 44

NH,
co NgY0
OR

transisomer
Ac al and optically inactive facial-isomer

Geometrical isomers 2, Optical isomers 2, Stereoisomers =3

Fig. 2.47 Stereoisomers of [Coz(en) (NH2 XNO2)]
Facial and Meridional Isomers:
In facial isomers tthree identical donor atoms lie on the corners ofa triangular face and in meridion OR
isomers three identicalcal donor atoms lie on the cormers of a plane bisecting the complex. There are fo
types uf complexes which exist in fac- and mer-isomeric forms. CO
NH, 0 gly
(3) [Mag bs]"* type omplexes :
Some examples of this type of complexes are :(Co(NH,);C, -(Co(NH, )s (NO2); CHa
[Cr(NH,), C, Jetc. Facial (or cis-) and meridional (or trans) isomers of(Co (NH3 )3Cis Jare showni co
Fig. 2.48. CA
meridiona-isomer

0 j0 Fia. 2.49 facial and mer- isomers of [Co (gykldn odt oidc
face and the three O-donor
In facial isomer three N-donor atoms occupy the corners of a trigonal
we can say that in thefacial isomer,
atoms occupy the corners of another trigonal face and in other words, N Co Nbond angles are
NH,
the three possible NCo Nbond angles are 90. In meridional isomer two
C 90° and one is
facial-isomer
meridional-isomer (c) The complex (Co (dien) (NO2)3] (Where dien NH2 -CH2 -CH2 -NHCH2 -CH2
Fig. 2.48 fac- and mer-isomers of (Co(NH3 )hClh]. -NH2) also exists in facial and meridional isomeric forms as shown in Fig. 2.50.
 Structl
2.32
cH. NH
NH
NH NH
CH,be
-CH,o -NH, H,NG
NH,
NO, NH
facialisomer
buluttal meridional-somerSalmo
(NO2a).ricisieO)lo (a)
Fig. 2.50 fac- and mer isomers of (Co (dien)
() The complex [Co (dien) also exists as facial- and meridional- isomers as shown in Fig. 251
134
3+

NH, SCH, H,N

NH (b

NHCH Fig. 2.52 Stretching of (a) Co Cl bond in [Co(NH3)4 Cl¡l' and

CH
NH, TNHH, (b) P-CI bond in (P(NH3)hClal
CH, (iD) Chemical Method : Grinberg's Method :
facial-isomer meridional-somer When achelating ligand such as NH,CH,COOH, COOH, NH)CH,-CH)-NH, etc. reacts
Fig. 2.51 facial and mer- isomers of [Co (ien) b ion.
In facial isomers the three COOH
three possible NCo-N bondN-donor
atoms
ofa dien ligand lie on the corners ofa trigonal face i.e., the
angles for a dien
bond angles and one N CoNbond angle for aligand are 90. In a meridional isomer two with cis- and trans- isomers separately, the cis -isomer forms a chelated complex or chelate and the trans
dien ligand are 90 and 180° respectively.NCo-N -isomer form a non-chelated complex. The reactions of, for example, cis- and trans - (Pt (NH3)2Cl,
To Distinguish cis- and with C,0 and NH,CH,CO0 are shown in Fig. 2.53.
The cis- and trans- isomers trans-Isomers
can be Quantitatively it is observed that for one mole of cis - isomer, one mole of NH,CH,COOH or
() Dipole Moment distinguished by the following methods (COOH)2 is used whereas for one mole of trans- isomer two moles of either NH,CH,COOH or
of oneMeasurements
the dipole moment : The trans- isomers has
dipole moments equal to (COOH)2 is used.
M-L bond is cancelled by the
opposite side. In cis-isome the dipole dipole moment of other M-L bondzero because
Iying on the cis- isomer forms chelate with chelating ligands because the two donor atoms coordinate on adjacent
moments
contribute in the same d irection resulting in a of two M-L bonds (which are cis- to one
(i) Infrared some value of dipole moment. another) positions resulting in less strain. But in case of trans- isomer, the donor atoms of chelating ligands can
vibrations, there mustSpectroscopy : In order for a
be a change in the dipole molecule to absorb infrared radiations to not coordinate at the trans- positions bacause of large strain. This large strain causes the unstability of
the chelated complex.
complexes such as [Pt (NH3);Cl2 Jand moment of the molecule as it vibrates. In case of cause
trane
no change in the dipole moment of
the (Co(NH})4Cl2],the Cl-metal-Cl symmetric stretching causes
inactive and no band corresponding tomolecule (see Fig. 2.52). Therefore, these twot
symmetric streching vibrations is
spectrum. In case of the cis - isomers of these compounds
are IR
vibrations of Cl-metal-C1 bonds complexes both the symmetriccand observed in the
infrared
vibrations for cis- complexes are IRcause appreciable changes in dipole
active and there are a number of moment. Thus, asymmertric stretching
bands in the these mode of
infrared spectrum.