6.4.1: Geometric Isomerism

Geometric Isomers are isomers that differ in the arrangement of the ligands around the metal or the central atom. In other words, these isomers differ from each other based on where the ligands are placed in the coordinate compound. This will be much easier to understand as examples will be considered. There are 2 main types of geometric isomers:

• Cis-Trans Isomers (referencing the spatial arrangement of two species)
• Mer-Fac Isomers (referencing the spatial arrangement of three species)

Identical Complexes

Because all vertices of a square are equivalent, it does not matter which vertex is occupied by the ligand B in a square planar MA₃B complex; hence only a single geometrical isomer is possible in this case (and in the analogous MAB₃ case). All four structures shown here are chemically identical because they can be superimposed simply by rotating the complex in space:

Octahedral complexes also exhibit cis and trans isomers. Like square planar complexes, only one structure is possible for octahedral complexes in which only one ligand is different from the other five (MA₅B). Even though we usually draw an octahedron in a way that suggests that the four “in-plane” ligands are different from the two “axial” ligands, in fact all six vertices of an octahedron are equivalent. Consequently, no matter how we draw an MA₅B structure, it can be superimposed on any other representation simply by rotating the molecule in space. Two of the many possible orientations of an MA₅B structure are as follows:

 

Cis-Trans Isomers

Cis-Trans Isomers are isomers that differ in the arrangement of two ligands in square planar and octahedral geometry. Cis isomers are isomers where the two ligands are 90 degrees apart from one another in relation to the central molecule. This is because Cis isomers have a bond angle of 90^o, between two same atoms. Trans isomers, on the other hand, are isomers where the two ligands are on opposite sides in a molecule because trans isomers have a bond angle of 180^o, between the two same atoms. When naming cis or trans isomers, the name begins either with cis or trans, whichever applies, followed by a hyphen and then the name of a molecule. For example a cis isomer of CoCl₂F₂ would be called cis-CoCl₂F₂. Finally, the last thing to keep in mind when examining cis and trans isomers is that only square planar and octahedral geometries can have cis or trans isomers. Examples of both isomers are provided below.

Square Planar

For an MA₂B₂ complex, there are two possible isomers: either the A ligands can be adjacent to one another (cis), in which case the B ligands must also be cis, or the A ligands can be across from one another (trans), in which case the B ligands must also be trans. Even though it is possible to draw the cis isomer in four different ways and the trans isomer in two different ways, all members of each set are chemically equivalent. Because there is no way to convert the cis structure to the trans by rotating or flipping the molecule in space, they are fundamentally different arrangements of atoms in space.

Example \(\PageIndex{1}\): cis Isomer

CoCl₂F₂:

(Color scheme: pink=cobalt, blue=fluorine, green=chlorine)

This above is an example of the molecule cis-CoCl₂F₂ or cis-dichlorodifluorocobalt (IV). The molecule pictured above is a cis isomer because both fluorine and chlorine ligands, respectively, are on the same side of the molecule. Additionally, one can approximate that the bond angle between each of the chlorine atoms and between each of the fluorine atoms is 90^o.

Isomer

CoCl₂F₂:

(Color scheme: pink=cobalt, blue=fluorine, green=chlorine)

(Note, the differences in the length of the bond between the two pictures are not intentional and have nothing to do with cis-trans isomerism)

This above is an example of the molecule trans-CoCl₂F₂ or trans-dichlorodifluorocobalt (IV).

We know the above molecule is a trans isomer because the two same chlorine atoms and the two same fluorine atoms are opposite each other. Furthermore, the bond angle between the two chlorine atoms and between the two fluorine atoms is 180^o. The above examples were all for square planar geometry but as the examples below illustrate, cis-trans isomerism can also occur in octahedral geometry. Both the molecules below are isomers of the molecule SCl₂F₄ (color scheme: yellow=sulfur, blue=fluorine, green=chlorine).

Example \(\PageIndex{3}\): Octahedral cis Isomer

SCl₂F₄:

We know this isomer above is a cis isomer because both the chlorine ligands are on the same side and the bond angle between the chlorine atoms appears to be 90^o.

Example \(\PageIndex{4}\): Octahedral trans Isomer

SCl₂F₄:

The isomer above is a trans isomer because the chlorine ligands are on opposite sides and the bond angle between the chlorine atoms is 180^o. All other isomers are essentially just rotations of these two isomers. Once again when trying to find cis and trans isomers look at the arrangement of the ligands. If two same ligands are on the same side, it is a cis isomer and if the ligands are on opposite sides, it is a trans isomer. Another way to tell the isomers apart is the bond angles: cis isomers have a 90^o bond angle whereas trans isomers have a 180^o bond angle.

Cis-Platin

Probably the best-known examples of cis and trans isomers of an MA₂B₂ square planar complex are cis-Pt(NH₃)₂Cl₂, also known as cisplatin, and trans-Pt(NH₃)₂Cl₂, which is actually toxic rather than therapeutic.

The anticancer drug cisplatin and its inactive trans isomer. Cisplatin is especially effective against tumors of the reproductive organs, which primarily affect individuals in their 20s and were notoriously difficult to cure. For example, after being diagnosed with metastasized testicular cancer in 1991 and given only a 50% chance of survival, Lance Armstrong was cured by treatment with cisplatin.

Square planar complexes that contain symmetrical bidentate ligands, such as [Pt(en)₂]²⁺, have only one possible structure, in which curved lines linking the two N atoms indicate the ethylenediamine ligands:

Octahedral

If two ligands in an octahedral complex are different from the other four, giving an MA₄B₂ complex, two isomers are possible. The two B ligands can be cis or trans. Cis- and trans-[Co(NH₃)₄Cl₂]Cl are examples of this type of system:

Example \(\PageIndex{1}\)

Draw all the possible geometrical isomers for the complex [Co(H₂O)₂(ox)BrCl]⁻, where ox is ⁻O₂CCO₂⁻, which stands for oxalate.

Given: formula of complex

Asked for: structures of geometrical isomers

Solution

This complex contains one bidentate ligand (oxalate), which can occupy only adjacent (cis) positions, and four monodentate ligands, two of which are identical (H₂O). The easiest way to attack the problem is to go through the various combinations of ligands systematically to determine which ligands can be trans. Thus either the water ligands can be trans to one another or the two halide ligands can be trans to one another, giving the two geometrical isomers shown here:

In addition, two structures are possible in which one of the halides is trans to a water ligand. In the first, the chloride ligand is in the same plane as the oxalate ligand and trans to one of the oxalate oxygens. Exchanging the chloride and bromide ligands gives the other, in which the bromide ligand is in the same plane as the oxalate ligand and trans to one of the oxalate oxygens:

This complex can therefore exist as four different geometrical isomers.

 

Mer-Fac Isomers

Mer-Fac isomers are easier to notice than cis-trans isomers in the sense that they only exist in octahedral geometry. Just like cis-trans isomers, mer-fac isomers are determined based on whether or not the ligands exist on the same side. Instead of dealing with 2 ligands, mer-fac isomers deal with 3 ligands. If the 3 ligands are all on the same side, the isomer is called a fac-isomer. Another way to identify fac isomers is to look at the bond angle between the ligands because fac isomers have a 90^o bond angle between each of the 3 atoms. The mer isomer on the other hand is where only 2 of the 3 ligands are on the same side. In mer isomers, there exists a 90^o-90^o-180^o bond angle between the 3 same ligands. In terms of nomenclature, mer-fac isomers follow the same rule as cis-trans isomers where you put the isomer type, followed by a hyphen, followed by the molecular formula. Examples have been provided below.

Replacing another A ligand by B gives an MA₃B₃ complex for which there are also two possible isomers. In one, the three ligands of each kind occupy opposite triangular faces of the octahedron; this is called the fac isomer (for facial). In the other, the three ligands of each kind lie on what would be the meridian if the complex were viewed as a sphere; this is called the mer isomer (for meridional):

Example \(\PageIndex{5}\): fac Isomer

Below is an example of the fac isomer, fac-CoCl₃F₃:

(note the color scheme: pink=cobalt, green=chlorine, blue=fluorine)

Through the 2d version, it is easier to see how the ligands are all on the same side. Nonetheless, in the 3D version, one can observe that the bond angle between the 3 same ligands is 90^o, thus making this isomer a fac-isomer.

Example \(\PageIndex{6}\): mer Isomer

Below is an example of the mer isomer, mer-CoCl₃F₃:

(note the color scheme: pink=cobalt, green=chlorine, blue=fluorine)

Its hard to tell in the 3D version but in the 2D version, one can easily tell how the same ligands are not on the same side. Additionally, one can approximate that the bond angle between the three chlorine atoms and between the three fluorine atoms is 90^o-90^o-180^o, thus making the above molecule a mer isomer.

Problems

 

Exercise \(\PageIndex{1}\)

Draw all the possible geometrical isomers for the complex [Cr(en)₂(CN)₂]⁺.

Answer

 

Two geometrical isomers are possible: trans and cis.

Contributors and Attributions

Prof. Robert J. Lancashire (The Department of Chemistry, University of the West Indies)