OCR Advanced GCE in Chemistry/Ligands and complexes

If you are not sure what a complex is or would like to recap, look here.

Using the concepts learnt in the trends and patterns module we can now develop those and look further into ligands and complexes

Complexes and shape

For the trends and patterns module we learnt about complexes consisting of ligands donating a lone pair of electrons to a central metal ion. This is called a monodentate ligand, meaning that each of the ligands donate one lone pair of electrons. The total number of coordinate bonds made by the donating of electron pairs is called the coordination number and the overall charge is a combination of the ligand charges and the metal ion charge. In this module it is required to know more about the different types of ligands which make up complexes:

Monodentate ligands - Donate one pair of electrons to the central metal ion e.g. H₂O, NH₃, CO, Cl^-

Bi dentate ligands - Donate two pairs of electrons to the central metal ion e.g. NH₂CH₂CH₂NH₂ - ethane-1,2-diamine (en)

If a ligand is donating more than one pair of electrons it will usually be known as polydentate e.g. en, EDTA

ethane-1,2-diamine is usually shortened to en when it is a ligand (this is acceptable in exams). (Looking right) There are two ammine groups (blue) that each have a lone pair of electrons. They both donate electrons in complexes so one molecule of en will form two coordinate bonds, hence bidentate. en can be put in a complex as any other ligand:

e.g. [Ni(en)₃]²⁺

When the coordination number is four there are two different shapes:

Square planar

Tetrahedral

When the coordination number is six the complex is octahedral

Isomerism

As with the organic chemistry modules, in complexes there is stereoisomers and optical isomers:

Stereoisomerism

In octahedral complexes (with 4 of one ligand and 2 of another) and square planar complexes (with 2 of one ligand and two of another) there are two different arrangements of the same atoms with the same bonds. These different arrangements are called cis and trans:

In cis molecules the 2 ligands are on the same side of the complex. This means that in both octahedral and square planar complexes the bond angle between the two like ligands will be 90 degrees.

In trans molecules the similar ligands are on the opposite sides of the molecules so the bond angle is 180 degrees.

Tetrahedral molecules do not show stereoisomerism

Cis-platin - This chemical is used as an anti-cancer drug because it can bind to the DNA of fast growing cells and prevent correct replication. The trans form of this chemical is not as effective at binding to the DNA so it is very important that the cis form is used.

Optical Isomerism

You should be familiar with the idea of optical isomerism from AS organic chemistry. When two molecules have the same atoms and bonds but are a mirror image of each other, they are not super imposable and are said to be optical isomers.

If you shine light through pure solutions of each optical isomer they would rotate the plane of polarised light in opposite directions. When they are together this effect is cancelled out.

Questions

1. What is a monodentate ligand?

2. How many coordinate bonds does an en ligand form in a complex?

3. What is cis-platin used for? why not trans?

4. Draw a the shape of a tetrahedral complex

5. What is the bond angle between two of the same ligand if there are 4 different ligands in a trans octahedral complex?

6. If i have two samples of a complex how can i test to find if they are the same optical isomers?

Answers