CoCl3⋅6NH3 CoCl3⋅5NH3 CoCl3⋅4NH3
Alfred Werner’s Experiments (~1890)
Cl
Cl
Cl
Cl
Cl
3 2 1free
Cl- ions :
total
# ions: 4 3 2
1
2
hexaamminecobalt(III) chloropentaamminecobalt(III)cis-dichlorotetraamminecobalt(III)
trans-dichlorotetraamminecobalt(III)
+ +
Coordination complexes: isomers
Different composition!
(covered in CEM311)
(not covered in CEM311)
but need to know it exists
If the stereoisomers are not mirror images
of one another, they are called:
geometrical isomers
or diastereoisomers
Stereoisomers that are mirror images of
one another are called:
optical isomers
or enantiomers
Geometrical Isomerism
“cis” “trans”
Same ligands around the metal, but different arrangement
Example 1: [Co(NH3)4Cl2]+
VIOLET GREEN
Barnett Rosenberg (1926-2009)
Michigan State University (1961-1997)
cis-dichlorodiammineplatinum
“cisplatin”
used to treat several
types of cancers:
testicular, ovarian, bladder, lung…
trans-platin is inactive
Discovered here at MSU in 1965 and still brings in millions $/yr in royalties!
Geometrical Isomerism
“fac” “mer”
Same ligands around the metal, but different arrangement
Example 2: [Co(NH3)3Cl3]
Optical isomers: enantiomers
Enantiomers are mirror images which are not superimposable
Enantiomers do not have a plane of symmetry
Any molecule which possesses a plane of symmetry is
superimposable on its mirror image
Enantiomers rotate polarized light in different directions; therefore,
enanotiomers are also termed “optical isomers”
Mirror images are either superimposible or they are not
Optical IsomerismA special case of geometrical isomerism:
same arrangements, but mirror images of each other
Example 3: [Co(ethylenediammine)3]3+
ethylenediammine = NH2CH2CH2NH2
Optical Isomerism
Criterion: two optical isomers CANNOT be superimposed,
no matter how you rotate them.
Like two gloves: cannot fit a right hand in a left-handed glove.
Optical Isomerism
Criterion: two optical isomers CANNOT be superimposed,
no matter how you rotate them.
Like two gloves: cannot fit a right hand in a left-handed glove.
Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on its mirror image
Two chiral structures: non superimposable mirror images:
Enantiomers!
StructureMirror image
Of structure
Plane of symmetry
Achiral (one structure)
Chirality: the absence of a plane of symmetry
*Enantiomers are possible
A molecule possessing a plane of symmetry is achiral and is
superimposible on its mirror image
*Enantiomers are NOT possible
No plane of symmetry
Chiral (two enantiomer)
NH3
CoH2O
H2OCl
Cl
NH3
NH3
CoCl
ClH2O
H3N
H2O
NH3
CoNH3
H2OCl
Cl
H2O
Are the following chiral or achiral structures?
How do we determine stability?
Formation Constant: Kf
ΔG = -RTlnKf
ΔG = ΔH – TΔS
f
M L ML
MLK
M L
Thermodynamic stability requires:
Kf > 1, ΔG < 0
i.e. products are more stable than reactants
Chelate Effect
Ni
NH3
NH3
NH3
NH3
NH3
NH3
2+
Ni2+
6 NH3
+
K = 108.6
Ni
NH2
N
NH2
NH2
NH2
NH2
2+
Ni2+
3 H2NCH2CH2NH3
+
K = 1018.3
H2