Coordination Bonds
- Transition element ions can form complexes which consist of a central metal ion and ligands
- A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons to the bond
- This is also the definition of a Lewis base
- This means ligands have a negative charge or a lone pair of electrons capable of being donated
- This definition may seem familiar: a ligand is the same as a nucleophile
- Different ligands can form different numbers of coordination bonds to the central metal ion in a complex
- Monodentate ligands can form one co-ordinate bond to the central metal ion
- Bidentate ligands can form two co-ordinate bonds
- Multidentate ligands can form multiple coordination bonds
- Coordination number is the number of co-ordinate bonds to the central metal atom or ion
Common Ligands
- Water molecules frequently act as ligands
- Each water molecule makes a single bond with the metal ion using one of the lone pairs on the oxygen atom
- The lone pair is donated to the partially filled d-subshell of the transition metal ion
Table showing examples of common monodentate ligands
| Name of Ligand | Formula of Ligand |
| Water | H2O |
| Ammonia | NH3 |
| Chloride | Cl– |
| Cyanide | CN– |
| Hydroxide | OH– |
Representing complex ions
- Square brackets are used to group together the ligands and metal ion in a representation of the geometrical arrangement
- The overall charge on the complex ion is the sum of the oxidation states of all the species present
- If the ligands are neutral then the overall charge will be the same as the oxidation state of the metal ion
Examples of complexes with monodentate ligands
Each ligand has one lone pair of electrons so forms one coordinate bond to the central metal ion
Coordination number
- The coordination number is the number of coordinate bonds to the metal ions
- This number can be the same as the number of ligands if they are monodentate
- It can be different if bidentate or multidentate ligands are present
- Complexes with a coordination number of 4 commonly have a tetrahedral shape, while complexes with a coordination number of 6 have an octahedral shape
Charges of complex ions
- The charge of a complex ion depends upon three factors:
- The charge of the central metal ion
- The charge of the ligands
- The coordination number
- If these factors are known then we can deduce the charge on the complex ion
Table showing the charges of complex ions
| Central ion | Ligand | Coordination number | Charge on complex ion | Formula of complex ion |
| Cu2+ | Cl– | 4 | 2+ + (4 x 1–) = 2– | [CuCl4]2– |
| Fe2+ | H2O | 6 | 2+ + (6 x 0) = 2+ | [Fe(H2O)6]2+ |
| Fe3+ | H2O | 6 | 3+ + (6 x 0) = 3+ | [Fe(H2O)6]3+ |
| Ag+ | NH3 | 2 | 1+ + (2 x 0) = 1+ | [Ag(NH3)2]+ |
Worked example
The three formulas shown are compounds of chromium(III). What are the charges on the complex ions shown?
I [Cr(H2O)6]Cl3
II [CrCl(H2O)5]Cl2.H2O
III [CrCl2(H2O)4]Cl.2H2O
A 0 , 0 , 0
B 1+, 2+, 3+
C 2+, 3+, 1+
D 3+, 2+, 1+
Answer:
- The correct option is D
- Water molecules do not contribute to the charge
- The chloride ion, Cl-, outside the square brackets must balance against the charge on the complex
- [Cr(H2O)6]Cl3 contains three chloride ions
- So, the charge on the complex is 3+
- [CrCl(H2O)5]Cl2.H2O contains two chloride ions outside the square bracket
- So, the charge on the complex is 2+
- [CrCl2(H2O)4]Cl.2H2O contains one chloride ion outside the square bracket
- So, the charge on the complex is 1+
- [Cr(H2O)6]Cl3 contains three chloride ions
Bidentate ligands
- Bidentate ligands can each form two co-ordinate bonds to the central metal ion
- This is because each ligand contains two atoms with lone pairs of electrons
- Examples of bidentate ligands are:
- 1,2-diaminoethane (H2NCH2CH2NH2) which is also written as ‘en’
- Ethanedioate ion (C2O42- ) which is sometimes written as ‘ox’ (coming from the common name of oxalate)
Transition metal complexes with bidentate ligands diagram
Each ligand has two lone pairs of electrons so forms two coordinate bonds to the central metal ion
Multidentate ligands
- Some ligands contain more than two atoms with lone pairs of electrons
- These ligands can form more than two dative bonds and are said to be multidentate or polydentate ligands
- An example of a multidentate ligand is EDTA4-, which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion
- EDTA comes from ethylenediaminetetraacetic acid, which is rather a mouthful so EDTA is easier!
Transition metal complexes with multidentate ligands diagram
The ligands contain more than one atom with a lone pair of electrons so multiple coordinate bonds are formed