5.3: Isomerism

This section will be about isomerism in coordination compounds. First, let us first briefly review the definition of isomerism: Isomerism is when two or more compounds have identical empirical formulas but different structures.

Definition: Isomerism

When two or more compounds have identical empirical formulas but different structures they are called isomers.

We distinguish between two basic types of isomerisms: Constitutional isomerism and stereoisomerism. What is the difference? In constitutional isomerism the bonds are not between the same atoms.

Definition: Constitutional Isomers

In constitutional isomers the bonds are not between the same atoms.

In stereoisomerism, the bonds are between the same atoms, but ligands are at different coordination sites.

Definition: Stereoisomers

In stereoisomers, the bonds are between same atoms, but the ligands are at different coordination sites.

Forms of Constitutional Isomerism

Hydrate Isomerism

First, let us look more closely at hydrate isomerism. In hydrate isomerism, in one isomer a water ligand is in the first coordination sphere, and in the other isomer it is in the second coordination sphere. A water molecule in the second coordination sphere is only defined for the solid state, but not in solution. This is because in solution the water molecule would become part of the solvent. Here are a few examples for hydrate isomers.

Figure \(\PageIndex{1}\) Two hydrate isomers of the complex [Cr(H₂O)₆]Cl₃

In the complex [Cr(H₂O)₆]Cl₃ there are six aqua ligands in the first coordination sphere (Figure \(\PageIndex{1}\)). In one hydrate isomer, there are only five water molecules in the first coordination sphere, the sixth one is in the second coordination sphere. One chloride anion has moved from the second to the first coordination sphere. There is a third hydrate isomer which has four aqua ligands in the first coordination sphere and two water molecules in the second coordination sphere. Two chloro ligands are now in the first coordination sphere, and one in the second coordination sphere. More hydrate isomers would be possible theoretically, but for some reason nature does not make them.

Figure \(\PageIndex{2}\) The hydrate isomers of the complex [Co(NH₃)₄(H₂O)Cl]Cl₂

Another example is [Co(NH₃)₄(H₂O)Cl]Cl₂. It has a hydrate isomer in which one aqua ligand has moved to the second coordination sphere, and one chloride anion has moved from the second to the first coordination sphere ( Figure \(\PageIndex{2}\) ).

Ionization Isomerism

A second form of isomerism is the ionization isomerism. In this case an ion moves from the first to the second coordination sphere and/or vice versa.

Figure \(\PageIndex{3}\) Two ionization isomers of the complex [Co(NH₃)₅NO₃]SO₄

For example, in the compound [Co(NH₃)₅NO₃]SO₄ there is a nitrate ion in the first coordination sphere, and a sulfate ion in the second coordination sphere ( Figure \(\PageIndex{3}\) ). There is an ionization isomer to this compound, in which the sulfate ion is now in the first coordination sphere, and the nitrate is in the second coordination sphere.

Coordination Isomerism

Another form of constitutional isomerism is coordination isomerism. In this isomerism, ligands are bound to different metal ions. Naturally, this implies that this form of isomerism can only exist if at least one isomer has two distinguishable metal ions.

Figure \(\PageIndex{4}\) Example of two coordination isomers with metal ions of different elements

In the example shown ( Figure \(\PageIndex{4}\) ), the first isomer has ethylenediamine bound to Co and cyanide bound to Cr, whereas in the second isomer ethylenediamine is bound to Cr and the cyano ligand is bound to Co.

Figure \(\PageIndex{5}\) Two coordination isomers with metal ions in different oxidation states (Oxidation states noted below each coordination sphere)

Further, in coordination isomers metal ions can be in different oxidation states. We can see that in the two depicted isomers there are complex cations, and complex anions with Pt in in different oxidation states ( Figure \(\PageIndex{5}\) ). In the first isomer Pt(II) makes the complex cation having four ammine ligands attached to it. Pt(IV) is part of the complex anion being surrounded by six chloro ligands. In the second isomer Pt(IV) forms the complex cation having four ammine and two chloro ligands in the first coordination sphere, and Pt(II) forms the complex anion having four chloro ligands.

Linkage isomerism

Linkage isomerism, also called ambidentate isomerism, is an isomerism that can be observed for ligands that have more than one reactive end. In two linkage isomers, the ligands will bind with different ends to the metal. Which end is reactive depends on the effective HOMO-LUMO interactions. Soft donor atoms tend to bind to soft metals, and hard donor atoms tend to bind to hard metals. Also the solvent can play a big role.

Figure \(\PageIndex{6}\) Two ambidentate isomers of thiocyanate

An example of an ambidentate ligand is the thiocyanate anion ( Figure \(\PageIndex{6}\) ). It can bind either with the sulfur or with the nitrogen end to a metal ion. When it binds with the S-end it is called the thiocyanato-ligand, when it binds with the N-end it is called the isothiocyanato ligand. Which atom binds to the metal can depend on the solvent. In the example shown, thiocyanate binds with S to Pd in polar solvents, but with N in apolar solvents. We could try to rationalize why. A possibility is that in polar solvents the more electronegative N atom can engage in hydrogen bonding which is not possible in apolar solvents. Steric arguments could also play a role. You can see that the triphenyl arsine ligands are fairly bulky. When the ligand binds with the nitrogen, then it binds in linear fashion avoiding steric interference with one of the arsine ligands. So it may be that in apolar solvents steric interactions dominate the behavior, while in polar solvents solvent-ligand interactions are in control.

It is even possible that two, same ambidentate ligands bind with opposite ends to the metal in one and the same molecule.

Figure \(\PageIndex{7}\) An example of two, same ambidentate ligands binding with opposite ends to the metal in one and the same molecule.

An example is the complex shown (Figure \(\PageIndex{7}\)). In this molecule, there is a thiocyanato and an isocyanato ligand binding to Pd. What arguments would we have to explain this ambidentate isomerism? Think about it for a moment. We can see that the two methyl groups are far less bulky than the two phenyl groups. When thiocyanate binds with the S-atom, then it can bend away from the two bulky phenyl groups. The second thiocyanate anion binds with the N atom because there is no significant interference between the methyl groups and the linear isothiocyanate ligand. This behavior indicates that thermodynamically, the Pd-N interaction is stronger, but only a little bit stronger because other factors such as steric interference can easily reverse the behavior.

Thio- and isothiocyanato ligands are not the only examples of ambidentate ligands.

Figure \(\PageIndex{8}\) Two linkage isomers of the nitrite anion

Another example is the nitrite anion. It can either bind with the N- end or the O-end to a metal. In the first case it is called a nitroisomer, in the latter it is called a nitrito isomer (Figure \(\PageIndex{8}\)). Nitritoisomers are usually more stable.

Stereoisomers (Configuration Isomers)

Now let us discuss the second major type of isomerism: Stereisomerism. As mentioned previously, in stereoisomerism the bonds are between the same atoms, but the positions at which the ligands bind, the coordination sites, are different. For the time being, we will only examine simple types of isomers, which are commonly called Geometric Isomers. Note that although the term Geometric Isomers is not IUPAC approved, it is commonly used by practicing chemists and is a handy way means of starting to learn about these concepts.

There is another class of stereoisomerism (Optical Isomerism) that we will not be studying in this course. Once you have learned about Optical Isomerism in simpler (typically carbon-based) molecules, then those same concepts can be applied to the sometimes more complicated coordination complexes in a future course.

Cis-Trans Isomerism

Let us now discuss some common forms of stereoisomerism, in this case Geometric Isomerism. The cis-trans isomerism is one very common stereoisomerism. It occurs when two, same ligands are in adjacent or opposite positions. For example, in a square planar complex two ligands can be adjacent or in opposite positions. When in adjacent position, the bond angle is 90° and we have a cis-isomer, when in opposite position, the bond angle is 180° and we have a trans-isomer.

Figure \(\PageIndex{9}\) Cis-platinum and trans-platinum, respectively

The probably most well known example of a cis-isomer in coordination chemistry is cis-platinum which is an anti-cancer drug (Figure \(\PageIndex{9}\)). Its trans-isomers does not have these pharmaceutical properties showing that cis-trans isomerism can have a profound impact on the properties of a molecule. Overall cis-trans isomerism in Pt(II) complexes have been most intensely studied, but cis-trans isomerism is also known for other d⁸ metal ions in square planar complexes. We can also ask if cis- and trans isomers are diastereomers or enantiomers. Let us look at the example of cis- and trans-platinum to answer this question.

Cis-trans isomerism extends beyond square planar complexes, and is also known for other shapes, for example, the trigonal bipyramidal shape, and the octahedral shape. In the cis-isomer of an octahedral complex two ligands occupy positions on the same face of the octahedron, whereas in the trans-isomer they occupy opposite position of the octahedron.

Figure \(\PageIndex{10}\) Cis and trans isomers of the diaquabromochlorooxalato cobalt(1-) complex, respectively

For example, in the complex diaquabromochlorooxalato cobalt(1-) there are cis and trans isomers known (Figure \(\PageIndex{10}\)). In the trans-isomer the two aqua-ligands stand in opposite position, and there is a 180° angle between them. In the cis-isomer they are in adjacent position, and the angle is 90°. We can see that the two ligands are on the same triangular face of the octahedron, shown in red.

Are there rules that can help us to decide if a cis- or a trans- complex will form? As you might suspect, the largest ligands usually go in trans-position due to steric repulsion arguments. Bidentate ligands usually form the cis-isomer because bidentate ligands are usually designed to make five- or six-membered rings with the metal ion.

Fac-mer Isomerism

Another common type of stereoisomerism in coordination chemistry is fac-mer isomerism. Fac stands for facial and mer stands for meridional. In a fac-isomer the same ligands are on a common face of a polyhedral complex, in the mer isomer they are on a plane that bisects the polyhedron. This kind of isomerism is very common for octahedral complexes, but not restricted to those.

Figure \(\PageIndex{11}\) Fac- (left) and mer-isomers (right) of the triammine trichloro cobalt(III) complex.

For example the complex triammine trichloro cobalt(III) ( Figure \(\PageIndex{11}\))has a fac- and a mer-isomer. You can see that in the fac-isomer the identical ligands are on two opposite triangular faces of the octahedron. In the mer-isomers they lie on two planes that bisect the octahedron.

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Dr. Kai Landskron (Lehigh University). If you like this textbook, please consider to make a donation to support the author's research at Lehigh University: Click Here to Donate.