Isomerism

! Two broad categories of isomers:structural isomers stereo isomers

! Structural or constitutional isomers – same empirical formula butdifferent molecular formulas or structural framework.• Types include ionization isomers, linkage isomers, coordination

isomers, and hydrate isomers.

! Stereo or configurational isomers – same structural framework butdifferent arrangements of ligands.• Types include optically active enantiomers (optical isomers) and non-

optically active diastereomers (geometrical isomers). • Stereo isomers that lack Sn, i, or F symmetry may occur in two

enantiomeric forms with opposite chirality.T Enantiomers are dissymmetric.T Enantiomers that have no symmetry (group C1) are asymmetric,

the least symmetric type of dissymmetry.

Classifying Isomers

CN4 Geometrical Isomers

! Tetrahedral CN4 complexes cannot have diastereomers, but squareplanar complexes with formulas MA2B2 and MA2BC can havecis/trans isomers, and MABCD complexes can have threegeometrical isomers

M

A A

B B

M

A

A

B

B

cis (C2v) trans (D2h)

M

A A

B C

M

A

A

B

C

cis (Cs) trans (C2v)

M

A D

C B

M

A D

B C

M

A C

B D

(Cs) (Cs) (Cs)

! Square planar complexes do not have optical isomers (enantiomers)unless caused by optically active ligands.

Octahedral CN6 Geometrical Isomers

! Octahedral MA6 (Oh) and MA5B (C4v) complexes do not havegeometrical isomers, but MA4B2 complexes have cis and trans forms.

M

A A

BA

B

A

M

A A

AA

B

B

cis (C2v) trans (D4h)

! MA3B3 has two geometrical isomers, fac (facial - C3v) and mer(meridional - C2v).

M

A B

BA

B

A

M

B A

BA

B

A

fac (C3v) mer (C2v)

• The fac isomer formerly was called cis, and the mer isomer formerlywas called trans.

• Complexes with tridendate chelates, such as [Co(dien)2]3+, can have

fac and mer isomers.

Co

N N

NN

N

N

Co

N N

NN

N

N

fac mer

Octahedral CN6 - More Complicated Cases

! Multidentate ligands and multiple ligands in the same complex canincrease the number and type of isomers greatly.

! Three classifications of geometrical isomers (", $, trans) have beenproposed for complexes of tren (triethylenetetramine) and similarligands.

Number and Type of Octahedral Isomers

Optically Active Tetrahedral Complexes

! Like asymmetric carbon compounds, MABCD tetrahedral complexeshave enantiomers.

A

M

CB

D

A

M

CB

D

C1

! Asymmetric chelates can create enantiomers in tetrahedral complexes.

M

A

A

BB M

A

A

BB

C2

Optically Active Octahedral Complexes

! Multidentate ligands can give rise to both diastereomers andenantiomers; e.g., [Co(en)2Cl2]

+.

Co

N N

NN

Cl

Cl

Co

N N

ClN

N

Cl

Co

N N

NCl

N

Cl

trans (D2h) cis (C1)

Labeling Chiral Octahedral Complexes

! IUPAC recommends using 7 and ) to indicate chirality of octahedralcomplexes.• Viewed down the C3 axis of the octahedral framework, a 7 isomer is

a left-hand screw.• Viewed down the C3 axis of the octahedral framework, a ) isomer is

a right-hand screw.

[Co(en)3]2+

Co

N

NNN

N NCo

N

NNN

NN

7 )left-hand screw right-hand screw

[Co(en)2Cl2]

Co

N

ClNN

N ClCo

N

NClN

NCl

7 )left-hand screw right-hand screw

Structural Isomerism

! Conformational – same coordination, but different geometry.

T NiCl2[N2P(CH2N]2 can be planar (~D2h) or tetrahedral (~C2v).

! Ionization – different distribution of ligand ions and counter ions.

T [Co(NH3)5Br]SO4 vs. [Co(NH3)5SO4]Br

! Hydrate – water as ligand vs. water as a hydrate in the crystal structure.

T [Cr(H2O)6]Cl3 [Cr(H2O)5Cl]Cl2@H2O [Cr(H2O)4Cl2]Cl@2H2Oviolet blue-green green

! Coordination – distribution of ligands between cation and anioncomplexes.

T [Co(NH3)6][Cr(CN)6] vs. [Cr(NH3)6][Co(CN)6]T [Cr(NH3)6][Cr(SCN)6] vs. [Cr(NH3)4(SCN)2][Cr(NH3)2(SCN)4]

! Linkage – mode of ligand attachment

M N

O

OM O N

O

nitro nitrito

M—SCN M—NCSS-thiocyanato N-thiocyanato

1H. J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, Inorg. Chem., 1977, 16, 2704; G.Rogez, S. Parsons, C. Paulsen, V. Villar, T. Mallash, Inorg. Chem., 2001, 3836.

Structural IsomerismPrussian Blue1

! The blue pigment Prussian blue is a unique example of linkageisomerism in the same compound.

! Prussian blue can be considered to be the first synthetic coordinationcompound.• The “soluble” form is KFe[Fe(CN)6].• The “insoluble” form is Fe4[Fe(CN)6]3.

! Prussian blue is a mixed-valence cyanide-bridged cubic networkcontaining low-spin FeII (S = 0) surrounded by carbon {FeII(CN)6} andhigh-spin FeIII (S = 5/2) surrounded by nitrogen {FeIII(NC)6}.• The cyanide bridges hold the structure together:

FeII—CN—FeIII

= FeII(CN)6 = FeIII(NC)6

Structural Isomerism

! Polymerization – same empirical formula but different molecularformulas.

T [Pt(NH3)2Cl2] vs. [Pt(NH3)4][PtCl4]