A complex compound contains a central metal ion that is surrounded by ligands - these are ions or molecules that donate a pair of electrons to the metals ion (co-ordinate bonding) and are therefore a Lewis base. The diagram below shows an example of a complex ion.
The example above is hexaamminecobalt ion. It consists of a cobalt (II) ion that has got 6 ammonia ligands bonded to it. This means it has a co-ordination number of 6. However, the co-ordination number is not always just the number of ligands, in fact this is only the case with unidentate ligands, meaning each ligand forms a singe bond to the metal ion (H2O, NH3 and Cl-).
Ligands can also be bidentate where they have two lone pairs to donate to the central atom. An example of a bidentate ligand is the ethanedioate ion, this has two seperate oxygen atoms with a free lone pair, so the species donates to bonds.
A multidentate ligand forms many co-ordinate bonds. An example is EDTA4- which uses all of its 6 donor sites to bind to the metal ion. Also, haem which is part of the protein haemoglobin (also hemoglobin), is an iron complex with a multidentate ligand.
Shapes of Complex Ions
Depending on the type of ligand, and the co-ordination number, the shape of the complex varies. The octahedral shape is the most common - this occurs when there are 6 ligands. The next most common is tetrahedral, this occurs in complexes with Cl- ligands. The reason is that only 4 of them can fit around the metal ion, so this is the arrangement they take.
And finally, silver (I) ions will form linear complexes. This is where one ligand is on either side of the silver ion with a 180° angle between them.
A property of transition metals is that they form coloured compounds, and this is also true of transition metal complexes. These colours are determined by: oxidation state, co-ordination number and the ligands. Therefore when we change these things, it results in a change of the compound's colour, see the example below.
Colour in solutions arises when a species absorbs visible light, meaning we see a combination of the remaining colours. The electron is excited from its normal to a higher energy state. This change in energy level is called ΔE.
Using light it is possible to determine the concentration of ion by looking at the intensity of the colour. To do this we use a spectrophotometer.
It works by passing visible and ultraviolet light of varying frequencies through the sample. The emergent light is detected. The amount of light absorbed is proportional to the concetration of the ion, therefore by knowing how much light is absorbed the concentration of the absorbing species can be determined.
In many compounds however, there is not much of a difference in the colours of varyingly concentrated compounds, so it is necessary to add another ligand in a substitution reaction. A substance that is used for this is bipyridyl (bipy), which is much more sensitive to changes in concentration.
Applications of Complex Ions
As you may know from biology, iron exists in the blood. The protein haemoglobin has an iron atom that is co-ordinately bonded to four nitrogen atoms that are part of larger molecules. Oxygen co-ordinates with the Fe2+ ion and can be transported. Carbon monoxide forms a more stable complex than oxygen, this means oxygen uptake is inhibited and carbon monoxide posioning can result.
A platinum complex ion is used in the anti-cancer drug Cisplatin: it consists of two ammonia and two chloride ligands on a platinium.
Different silver complexes also have practical applications.
|[Ag(NH3)2]+||Is used in Tollens' reagent which tests for aldehyde or ketones.|
|[Ag(S2O3)2]3-||Is formed in photography when silver bromide that hasn't been exposed to light is dissolved in sodium thiosulphate solution.|
|[Ag(CN)2]-||A complex formed when Ag salts are dissolved in potassium cyanide - this solution is used as the electrolyte in silver plating.|