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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