Transition Metal Complexes

A covalent bond is a shared pair of electrons Usually the two atoms involved in the covalent bond

each contribute one electron

Definition: In a co-ordinate bond, one atom donates both of the electrons in the shared pair of electrons

In order for a co-ordinate bond to form there must be a lone pair of electrons on one of the atoms

A co-ordinate bond is represented by an arrow, →

Definition: A complex is a central metal ion surrounded by ligands

Definition: A ligand is an ion or molecule that can donate a lone pair of electrons to the central metal ion

The ligand donates its lone pair of electrons to the metal ion forming a co-ordinate bond

The ligand must have a lone pair of electrons

Ligands can be negative ions such as:

Halide ions - Chloride ions, :Cl-, Bromide ions, :Br -

Hydroxide ion, :OH-

Cyanide ion, :CN-

Ligands can also be neutral molecules which contain more than one lone pair of electrons such as:

Ammonia, :NH3

Water, H2O:

The oxygen atom in water has two lone pairs of electrons

A ligand that can donate one lone pair of electrons is a unidentate ligand

A ligand that can donate two lone pairs of electrons is a bidentate ligand

A ligand that can donate more than two lone pairs of electrons is a multidentate ligand

Even although water has two lone pairs of electrons, it can only donate one pair of electrons

Therefore water is a unidentate ligandO

H H

: :

Definition: The co-ordination number is the number of pairs of electrons donated to the central metal ion

Co-ordination numbers are 2, 4 or 6

Complex Co-ordination number[Ag(NH3)2]+

[CuCl4]2-

[Cu(H2O)6]2+

[FeCl4]2-

[Cu(H2O)4(OH)2]

[Ag(H2O)2]+

246462

The central metal ion has a positive charge

The overall charge on the complex depends on the charge of the metal ion and the charge on the ligands

Example: [Cu(H2O)6]2+

Overall charge = 2+ Oxidation state of copper = +2

Remember water is a neutral ligand

Complex Overall charge Oxidation state of metal ion

[Ag(NH3)2]+

[Cu(H2O)6]2+

[CuCl4]2-

[V(H2O)6]3+

[Cu(H2O)4(OH)]

[Ag(CN)2]-

[NiCl4]2-

[CrCl2(H2O)4]+

1+ +1

2+ +2

2- +2

3+ +3

0 +2

1- +12- +2

1+ +3

Silver forms complexes with a co-ordination number of 2

The shape of the complex is linear

Example: Tollens’ reagent (Test for aldehydes)

Tollen’s reagent contains the complex, [Ag(NH3)2]+

There are two ammonia ligands each donating one lone pair of electrons to the Ag+ ion

[H3N: → Ag+ ← :NH3]+

When Tollen’s reagent is warmed with an aldehyde, the silver(I) ion (Ag+) is reduced to silver atoms (Ag) producing a silver mirror

Ketones do not reduce the silver(I) ion and no silver mirror is formed