The coordination number of a complex is the number of atoms making coordinate or dative covalent bonds to the central metal ion (not the number of ligands) and it helps determine the geometric shape of the complex. The most common coordination numbers are 4 and 6 and you need to be able to draw examples of these complexes in 3D.
Coordination number 4
Complexes with a coordination number of 4 have either a tetrahedral or square planar geometry.
In a tetrahedral complex the bond angle around the central metal ion is 109.5°. This geometry is the most common .
In a square planar complex the bond angle is 90°. This geometry is common in platinum(II), palladium(II) and gold(III) complexes.
Coordination number 6
Complexes with a coordination number of 6 have an octahedral geometry, with bond angles of 90°.
Practice questions
- Glycinate ions, NH2CH2COO–, form a complex with copper.
(a) Draw the structure of the glycinate ion.
(b) Use your answer to (a) to determine how many coordinate bonds the glycinate ligand can make to a metal ion.
(c) Complete the diagram below to show the structure of the complex ion formed between glycinate ligands and Cu2+.
(d) Name the shape of this complex ion and give the bond angles around the Cu2+.
- Name and explain the shape of the [Co(NH3)6]2+ complex ion using electron-pair repulsion theory.
Answers
- (a)
(b) glycinate can form two coordinate bonds – one using a lone pair of electrons on the nitrogen atom and one using the lone pair on the negatively charged oxygen
(c)
(d) square planar, the bond angles around the Cu2+ are all 90°
- The [Co(NH3)6]2+ complex ion has an octahedral shape. There are six ligands (NH3) surrounding the central metal ion and each ligand is making one dative covalent bond to Co2+ using the lone pair of electrons on the nitrogen atom. These bonding electron pairs move to minimise repulsion which results in an octahedral arrangement around the central metal ion.