A. Geometric Isomerism for Some Common Octahedral Complex Compositions

1. Composition MA₄B₂ has 2 geometric isomers, cis and trans. We have already seen in Lecture 6 that there are two geometric isomers possible for this composition when the coordination geometry is octahedral: cis and trans. The key is to not get confused by different ways of drawing these isomers. You should especially remember that all six positions on an octahedron are equivalent: there are not axial and equatorial positions as there are for a trigonal bipyramid.

For example, both drawings below represent the cis-tetraamminedichloroplatinum(IV) ion, in which the Cl-Pt-Cl bond angle is 90^o:

And both of these drawings represent the trans-tetraamminedichloroplatinum(IV) ion, in which the Cl-Pt-Cl bond angle is 180^o.

2. Composition MA₃B₃ has two geometric isomers, fac and mer.The fac- isomer (short for "facial") gets its name because all three chlorides are coordinated on one face of the octahedron. The mer-isomer (short for "meridional") has the three chloride ions coordinated in a plane that includes the metal ion. (This corresponds to the meridian on a sphere, which is any plane through the sphere that contains the center.)

fac-triamminetrichlorocobalt(III) All three Cl-Co-Cl bond angles are 90o.	mer-triamminetrichlorocobalt(III) Two Cl-Co-Cl bond angles are 90o, the other is 180o.

3. Composition MA₂B₂C₂ has five geometric isomers. It's just a matter of working out all the cis and trans possibilities for each pair of ligands. Consider the cobalt complex [Co(NH₃)₂Cl₂(NO₂)₂]^-.

cis-diammine-cis-dichloro-trans-di-N-nitritocobaltate(III) ion	trans-diammine-cis-dichloro-cis-di-N-nitritocobaltate(III) ion
cis-diammine-trans-dichloro-cis-di-N-nitritocobaltate(III) ion	cis-diammine-cis-dichloro-cis-di-N-nitritocobaltate(III) ion
trans-diammine-trans-dichloro-trans-di-N-nitritocobaltate(III) ion

4. Composition M(A-B)₃ has two geometric isomers, fac and mer. Here, A-B represents an unsymmetrical bidentate chelating agent such as the aminoethanethiolate ion, NH₂CH₂CH₂S^-, or the benzoylacetonate ion whose resonance forms are shown below. The donor atoms are different for aminoethanethiolate so it is clearly unsymmetrical. However, even though the donor atoms are same (both oxygens) for benzoylacetonate, the two "ends" of the chelating anion are different: one oxygen is close to a methyl group, the other to a phenyl group

resonance structures forbenzoylacetonate

fac-isomer, Co(bzac)3. Op is the oxygen closest to the phenyl group, Om is the oxygen closest to the methyl group.	mer-isomer, Co(bzac)3 . Here the Om's are all meridional, as are the Op's

5. Composition M(A-A)₂B₂ has two geometric isomers, cis and trans. Here, A-A represents a symmetrical bidentate ligand such as acetylacetonate, oxalate, or ethylenediamine. The isomers for [Co(en)₂Cl₂]⁺ are shown below.

cis-dichlorobis(ethylenediamine)cobalt(III) ion	trans-dichlorobis(ethylenediamine)cobalt(III) ion

6. M(trien)B₂ has 3 geometric isomers. Trien is a linear tetradentate ligand,

The flexible trien ligand can coordinate to an octahedral center in 3 different ways as shown below for [Cr(trien)Cl₂]⁺. Naming the complexes requires a method beyond the scope of CHEM 2P32. N_m and N_t designate the middle NH groups and the terminal NH₂ groups, respectively, on the ligand.

Isomer A: both chlorides are trans to the middle nitrogens	Isomer B: the chlorides are trans to each other	Isomer C: one chloride is trans to a middle nitrogen, the other is trans to a terminal nitrogen.

SELF-TEST 1: GEOMETRIC ISOMERS. Budotitane is a 6-coordinate octahedral titanium complex that has been developed for treating colon cancer. Its formula is [Ti(bzac)₂(OEt)₂], where bzac is the benzoylacetonate ligand shown above. The OEt ligand is ethoxide, CH₃CH₂O^-. Draw the possible geometric isomers. Answer.

SELF-TEST 2: GEOMETRIC ISOMERS. Draw the geometric isomers of the octahedral complex, [Co(dien)Br₂Cl]. dien is the linear tridentate ligand NH₂CH₂CH₂NHCH₂CH₂NH₂. Answer.

B. Optical Isomerism for Some Common Octahedral Complex Compositions

1. M(A-A)₃ has optical isomers. See the previous lecture (click here). This means that any octahedral tris chelate, whether with a symmetrical or an unsymmetrical bidentate ligand, is chiral and will have optical isomers.

2. cis-M(A-A)₂X₂ has optical isomers. It follows that cis-M(A-B)₂X₂ is chiral and will also has optical isomers. The optical isomers of [Co(en)₂Cl₂]⁺ are shown below.

3. The cis-cis-cis geometric isomer of MA₂B₂C₂ is chiral. The cis-diammine-cis-dichloro-cis-di-N-nitritocobaltate(III) ion, one of the 5 geometric isomers of [Co(NH₃)₂Cl₂(NO₂)₂]^- shown above, has no plane of symmetry in the ion and thus has optical isomers.

4. All 15 geometric isomers of the complex MABCDEF are chiral. It's unlikely that all 15 isomers with this composition would ever be synthesized! But any that are will be chiral and have optical isomers.

C. Geometric and Optical Isomerism for Some Tetrahedral and Square Planar Complexes

In the following examples we will contrast the number and types of isomers for tetrahedral vs. square planar complexes with a particular composition. The different number and kinds of isomers obtained for a given composition allowed a determination of geometry in the days before x-ray crystallography and other modern techniques were available to researchers.

1. Tetrahedral MA₂B₂ has no isomers, but square planar MA₂B₂ has geometric isomers. Four-coordinate platinum(II) complexes are always square planar, whereas 4-coordinate cobalt(II) complexes are tetrahedral. The cis and trans isomers of diamminedichloroplatinum(II) are shown below.

On the other hand, tetrahedral diamminedichlorocobalt(II) has no isomers, either geometric or optical. (As you would expect, given your familiarity with such carbon compounds as CH₂Cl₂).

2. Tetrahedral MABCD has optical isomers but not geometric isomers; square planar MABCD has geometric isomers but not optical isomers. In the sketch below, M can be any metal that exhibits square planar coordination. The three isomers place A trans to C, A trans to B, and A trans to D. None of these is chiral (the square plane is the plane of symmetry in these complexes).

Tetrahedral MABCD is analogous to carbon bonded to four different groups, and thus has optical isomers but not geometric isomers.

3. Neither tetrahedral M(A-A)₂ nor square planar M(A-A)₂ has isomers, as long as the chelating ligands are symmetrical and there are no chiral centers on the ligands.

4. Tetrahedral M(A-B)₂ has optical isomers but not geometric isomers; square planar M(A-B)₂ has geometric isomers but not optical isomers. Real examples of unsymmetrical A-B ligands are aminoethanethiolate and benzoylacetonate complexes. Aminoethanethiolate is NH₂CH₂CH₂S^-; see above for the benzoylacetonate structure. Consider the aminoethanethiolate complexes of square planar platinum(II) and tetrahedral cobalt(II).

geometric isomers of [Pt(NH2CH2CH2S)2]	optical isomers of [Co(NH2CH2CH2S)2]

This page is http://chemiris.labs.brocku.ca/~chemweb/courses/chem232/CHEM2P32_Lecture_7.html
Created January 18, 2001 by M. F. Richardson
© Brock University, 2001