d-block metal cations readily form complexes with ligands – neutral molecules such as water or anions such as the chloride ion that donate a pair of electrons to the central metal ion, essentially forming coordinate bonds / dative covalent bonds.
For example, when cobalt(II) nitrate dissolves in water, six water molecules coordinate to the Co2+ ion with each oxygen in a water molecule donating a lone pair of electrons to the central Co2+ ion.
The solution consists of [Co(H2O)6]2+ complex ions and nitrate ions.
Water is an example of a monodentate ligand because it donates 1 pair of electrons to a central metal ion, but there are many examples you should be familiar with.
| ligand | example |
|---|---|
| H2O: | [Cr(H2O)6]3+ |
| :NH3 | [Ni(NH3)6]2+ |
| :CO | [Ni(CO)4] |
| :F– | [TiF6]2- |
| :Cl– | [CuCl4]2- |
| :Br– | [NiBr4]2- |
| :OH– | [Fe(OH)H2O)5]2+ |
| :CN– | [Fe(CN)6]4- |
We can work out the oxidation state of the central metal ion in a complex ion if we know the charge on the ligands and on the complex.
Bidentate ligands donate 2 pairs of electrons to the central metal ion so that each ligand is forming two coordinate bonds.
E.g. 1,2-diaminoethane / ethane-1,2-diamine (en)
E.g. 1,3-diaminopropane / propane-1,3-diamine (pn)
E.g. ethanedioate ion / oxalate ion (ox)
It is also worth noting that a ligand can be used to bridge two metal ions.
A complex formed with polydentate ligands (those donating more than 1 pair of electrons) is also called a chelate and the polydentate ligand are sometimes known as chelating ligands, as seen in the examples shown above.
The ethylenediaminetetracetate ion, EDTA4-, is an example of a hexadentate ligand with coordinate bonds forming between the central metal ion and the lone pair of electrons on 4 oxygen atoms and 2 nitrogen atoms!
Practice questions
- Ni2+ ions form a complex ion with the ligand 1,2-diaminoethane, H2NC2H4NH2. Explain the term ‘complex ion’ and why 1,2-diaminoethane is considered to be a bidentate ligand.
- In a complex ion the central metal ion is surrounded by ligands. Explain why the ammonium ion cannot behave as ligand.
- Discuss the types of bonding found within a complex ion such as [Co(H2O)6]2+.
- Determine the oxidation number of silver in [Ag(CN)2]–, copper in [Cu(EDTA)]2- and vanadium in [V(OH)3(H2O)3]2+.
- Show how the ethanedioate ligand bonds to a Fe3+ ion on the diagram below. Label your answer with the type of bond formed.
Answers
- A complex ion consists of a number of ligands making dative covalent or coordinate bonds to a central metal cation. 1,2-diaminoethane is a bidentate ligand because it is able to form 2 dative covalent / coordinate bonds to Ni2+ using the lone pair of electrons on each of the two nitrogen atoms.
- The ammonium ion, NH4+, has no lone pair of electrons to donate to the central metal ion (the lone pair on the nitrogen atom is already making a dative covalent bond to a proton).
- There are covalent bonds between the oxygen and hydrogen atoms in each molecule of water that is behaving as a ligand, and dative covalent / coordinate bonds between the Co2+ ion and each water ligand using a lone pair of electrons from the oxygen atom.
- Ag has an oxidation number of +1 (the ligand is CN–); copper has an oxidation number of +2 (the ligand is EDTA4-); vanadium has an oxidation number of +5 (the ligands are OH– and H2O).
- Marks for lone pair of electrons on each oxygen, dative covalent bonds to Fe3+, label
