Coordination Compounds
(Metal Complexes)
A compound in which there is a central metal atom or ion surrounded by a number of ligands
attached to central metal atom or ion through coordinate covalent bonds is called a
coordination compound or a metal complex.
Examples- [Ag(NH3)2]Cl; K4[Fe(CN)6]; [ Cu(NH3)4]SO4; [Ni(CO)4]
Classification of Coordination Compounds
(Metal Complexes)
1- Neutral complexes
Examples; [Ni(CO)4], [Fe(CO)5], [Pt(NH3)2Cl2]
2- Cationic complexes
Examples; [Ag(NH3)2]+, [Pt(NH3)4] +2, [Fe(H2O)6] +3
3-Anionic complexes
Examples; [Ag(CN)2]-, [PtCl4] -2, [Fe(CN)6] -2, [Ni(CN)4]-2
Ligand
An atom, ion or molecule, which can donate an electron pair to a central metal atom or ion
through a coordinate covalent bond in a coordination compound, is called a ligand.
Examples; NH3, H2O, CO, Cl-, Br-, I-, NO2-, CN-, NO+
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Classification of Ligands
Ligands are classified by various methods.
1- On the basis of charge on the ligands
2- On the basis of number of electron pairs that it can donate
3- On the basis of Sigma donors and Pi acceptors
Classification of ligands on the basis of charge on the ligands
1- Neutral Ligands
NH3, H2O, CO, en
2- Negatively charged Ligands
Cl-, Br-, I-, NO2-, CN-, C2O4-2
3- Positively charged Ligands
NO+, H2N-NH3+
Classification of ligands on the basis of number of electron pairs
that it can donate
1- Mono dentate Ligands
NH3, H2O, CO, Cl-, Br-, I-, NO2-, CN-, NO+
2- Bi dentate Ligands
Ethylenediamine (en), Oxalate ion, 2,2.-bipyridine
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3- Tri dentate Ligands
4- Quadri dentate Ligands
5-Quinquadri dentate Ligands
Ethylene diamine tri acetic acid
6- Hexa dentate Ligands
Ethylene diamine tetra acetic acid (EDTA)
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Classification of ligands on the basis of charge on the ligands
1- Sigma donor ligands
2- Pi acceptor ligands
Sigma donor ligands
The ligands which can donate electron pair to the central metal atom or ion in a
metal complex, through sigma bonds but cannot accept electron density back
through pi bonds, are called Sigma donor ligands.
Examples; NH3, H2O, Cl-, Br-, I-, NO2-, NO+
Sigma donor ligands form coordination compounds with metals in high oxidation
states.
Pi acceptor ligands
The ligands which can donate electron pair to the central metal atom or ion in a
metal complex, through sigma bonds but can also accept electron density back
through pi bonds, are called Pi acceptor ligands.
Examples; CO, CN-, NO, Ethylene
Pi acceptor ligands form coordination compounds with metals in zero or low
oxidation states.
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Coordination Number
The number of electron pairs donated to the central metal atom or ion by the ligands in a
metal complex through coordinate covalent bonds is called Coordination Number.
Each metal ion has a definite coordination number in a particular oxidation state.
Metal ion Oxidation State Coordination Number
Ag+ +1 2
Ni+2 +2 4, 6
Fe+2 +2 6
Fe+3 +3 6
Pt+2 +2 4
Pt+4 +4 6
Coordination Sphere
The space around the central metal atom or ion in a metal complex, in which the ligands are
located, is call coordination sphere. It is denoted by a Square bracket [ ].
Examples.
[Ag(NH3)2]+ has Linear geometry.
[Pt(NH3)4] +2 has Square Planar geometry.
[Ni(CO)4] has Tetrahedral geometry.
[Fe(H2O)6] +3 has Octahedral geometry.
