Ligands and Complex Ions

Learning Objectives (OCR Spec 5.3.1)

By the end of this lesson you should be able to:

• Define ligand, complex ion, coordination number and dative covalent (coordinate) bond
• Identify common monodentate ligands: H2O, NH3, Cl-, OH-, CN-, CO
• Distinguish monodentate, bidentate and polydentate ligands
• Describe the shapes of complex ions with coordination numbers 2, 4 and 6
• Calculate the charge on a complex ion from the oxidation state of the metal and the charges on the ligands
• Use formulae like [Cu(H2O)6]2+ correctly (brackets, charge, subscripts)

Key Definitions

A ligand is a molecule or ion with a lone pair of electrons that can form a dative covalent bond to a central metal ion.

A complex ion (or complex) is a central metal ion surrounded by and bonded to one or more ligands by dative covalent bonds.

A dative covalent (coordinate) bond is a covalent bond where both shared electrons come from the same atom (the ligand), not one from each as in a normal covalent bond.

The coordination number is the number of dative covalent bonds (equivalently, the number of lone pairs donated to the metal) in the complex. For monodentate ligands, this equals the number of ligand molecules; for bidentate and polydentate ligands, it is more than the number of ligand molecules.

Writing Complex Ion Formulae

Complex ions are always written with:

• Square brackets around the whole complex
• The metal symbol first, then the ligands
• Anionic ligands before neutral ligands (alphabetical within each class in modern IUPAC; OCR is flexible)
• The overall charge as a superscript outside the brackets

Examples of the correct notation:

• [Cu(H2O)6]2+
• [CuCl4]2-
• [Ag(NH3)2]+
• [Fe(CN)6]3-
• [Cr(OH)6]3-

The oxidation state of the metal is not shown in the formula (you calculate it from the charge on the complex and the charges on the ligands).

Calculating Metal Oxidation State in a Complex

The overall charge on the complex equals the sum of the metal oxidation state and the charges on all ligands.

Worked examples:

• [Cu(H2O)6]2+: H2O is neutral, so 0 + x = +2, x = +2. Copper is in the +2 oxidation state.
• [CuCl4]2-: each Cl- is -1, so 4(-1) + x = -2, x = +2. Copper is +2.
• [Fe(CN)6]3-: each CN- is -1, so 6(-1) + x = -3, x = +3. Iron is +3 (ferricyanide, a classic reagent).
• [MnO4]-: each O2- is -2, so 4(-2) + x = -1, x = +7. Manganese is +7 (permanganate).
• [Co(NH3)6]3+: NH3 is neutral, so 0 + x = +3, x = +3. Cobalt is +3.

Common Monodentate Ligands

A monodentate ligand donates exactly one lone pair (has one donor atom that binds once).

Ligand	Formula	Donor atom	Charge
Water	H2O	O	0
Ammonia	NH3	N	0
Chloride	Cl-	Cl	-1
Hydroxide	OH-	O	-1
Cyanide	CN-	C	-1
Carbon monoxide	CO	C	0

These are the six monodentate ligands OCR expects you to know by name and formula. The most important are water (the default in aqueous solution), ammonia (common in substitution), chloride (makes large tetrahedral complexes) and cyanide (strong-field, coloured complexes).

Bidentate Ligands

A bidentate ligand has two donor atoms and forms two dative bonds from the same molecule. OCR expects you to know two examples:

• 1,2-diaminoethane, H2N-CH2-CH2-NH2 (abbreviation en): two nitrogen donor atoms
• Ethanedioate (oxalate), -O-CO-CO-O- or (COO-)2 (abbreviation ox or C2O4^2-): two oxygen donor atoms, charge 2-

A complex like [Ni(en)3]2+ contains three bidentate ligands, each donating two lone pairs - so the coordination number is 6 even though there are only 3 molecules.

Polydentate Ligands

A polydentate (or multidentate) ligand has more than two donor atoms and forms three or more dative bonds.

The most important polydentate ligand is EDTA4- (ethylenediaminetetraacetate). It has six donor atoms (2 N + 4 O-) and wraps around a single metal ion to fill all six coordination sites. An EDTA complex is therefore [M(EDTA)]^(n-4) where n is the charge on the metal.