Coordination Complex

[Co(NH3)6]Cl3

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?

FIGURE 7.67

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

Types of Ligands: Ethylenediaminetetraacetate ion

(EDTA): a polydentate chelating ligand

Chelate from

Greek chela, “claw”

EDTA wraps around the metal ion

at all 6 coordination sites producing

an exceedingly tight binding to the

metal