What are coordination compounds and how to classify them

Here in this article you will learn about the coordination compound, classification of ligands and examples of coordination compounds.

What is Coordination Compound ?

Coordination compounds are the compounds in which the central metal atom is linked to a number of ions or neutral molecules by coordinate bonds. Dative or coordinate bond is formed by the donation of lone pair of electrons by the neutral molecules or ions to the central metal atom. Coordination chemistry is the branch of inorganic chemistry dealing with the study of coordination compounds.

Examples of Coordination Compounds

For example in case of nickel tetra carbonyl [Ni(CO)4 ] CO molecules are linked to the central nickel atom by coordinate bonds by donating lone pair of electrons to form coordination compound.

If ions or neutral molecules are linked by coordinate bonds to the central metal atom or ion which carries positive or negative charge it is called a complex ion. For example [Fe(CN)6]- is a complex ion.

Classification of Complexes or Complex ions 

  1. A complex in which the complex ion carries a net negative charge is called an anionic complex  e.g. [Fe(CN)6]-
  2. A complex in which the complex ion carries a net positive charge is called a cationic complex  e.g. [Co(NH3)6]3+
  3. A complex getting more no net charge is called a neutral complex e.g. [NiCO]4

Difference between a double salt and a complex compound

Double saltsComplex compounds
1. Double salts usually contain 2 simple salts in equimolar proportion1. Complex compounds are formed by the simple salts which may or may not be in equilmolar proportions
2. They are ionic compounds and do not contain any coordinate bond2. Complex compound may or may not be ionic but the complex part always contains coordinate bonds
3. In double salts the metal ions show their normal Valency3. Here the metal ion satisfies its two types of Valency is called primary Valency and secondary valency
4. Double salt loses its identity in the solution4. Coordination compound or complex retain its identity in the solution
5. Properties of the double salt at same as that of its constituent compounds.5. Properties of the coordination compound are different from its constituents.
examples Mohr’s salt FeSO4(NH4)2SO4.6H2O , Potash alum K2SO4Al2(SO4)3.24H2Oexamples K4[Fe(CN)6] dissociates into 4K+  + [Fe(CN)6]4-   and the complex ion does not dissociate further

Coordination Compounds Class 12 Notes 

You can  also download the coordination compounds pdf class 12. You can also download multiple choice question series Chemistry Class 12 p block PDF notes from here MCQ’s Series #5 group 16 elements p block elements Class 12

Here is the video to check out the introduction and the classification of ligands from coordination compounds

Terms used in coordination compounds

Ligands and central metal atom/ion

Ligands may be defined as the ions or molecules which donate a pair of electrons to the metal atom or ion to form a coordinate bond. The metal atom or ion to which the ligands are attached is called as central metal atom or ion. For example in [Fe(CN)6]- Fe is central metal ion and CN is negative ligand. Ligands can also be called as Lewis bases whereas the central metal atom or iron is called as Lewis acid. 

Classification of ligands in coordination compounds

If the ligand atom contains only one donor atom in the molecule which can form coordinate bond then it is called as unidentate ligand. For example H2O (in water oxygen contains two lone pairs but it is a unidentate ligand because to act as a bidentate ligand and it must have two donor atoms),  NH3, CN -, OH-

If the ligand atom contains two donor atoms (coordinating groups ) forms five or six membered ring then it is called as bidentate or Didentate chelating ligand and the property is called as chelation. Examples like ethylenediamine has two nitrogen atoms and oxalate ion has also two oxygen atoms which can link to the central metal atom or ion.

Polydentate ligands

Among the polydentate ligands terdentate or tridentate, tetradentate, hexadentate types of ligands may be present in the coordination complexes

Denticity of ligand

The number of coordinating groups present in a ligand is called the denticity of the ligand.

Ambidentate ligand

Unidentate ligands containing more than one coordinating atoms are called as ambidentate ligands. For example Nitrito -N and Nitrito- O.  So, NO2 two can link through N or O or SCN can link through S or N, etc

Coordination number in coordination compounds

The number of coordinate bonds formed by the ligands with metal atom i.e. number of unidentate ligands or double the number of bi dentate ligands is called as coordination number for example in K4[Fe(CN)6] the coordination number of iron is 6, in [Ni(CO)4 ]  the coordination number of nickel is 4

Coordination entity and counter ions in coordination compounds

The part enclosed in square brackets containing metal atom linked to ligands is called coordination entity. And the ions present outside the coordination entity are called as counter ions.

Oxidation number or oxidation state in coordination compounds

The charge present on the metal atom in its complex is called its oxidation number or oxidation state. For example K4[Fe(CN)6] charge on potassium is +2 so its oxidation state is 2 and in nickel tetra carbonyl [Ni(CO)4 ] Ni has oxidation state equal to 0

Homoleptic and Heteroleptic complexes coordination compounds

Complexes which contain only one type of ligand are called as homoleptic complexes  for example [Fe(CN)6]4-.  And those which contain more than one type of ligand are called as heteroleptic complexes for example [Co (NH3)4 Cl2]+

Coordination polyhedron

The spatial arrangement of ligands around the central metal atom is called coordination polyhedron which may be tetrahedral, square or octahedral, square pyramidal or trigonal bipyramidal

Homonuclear and polynuclear complexes

Complexes in which only one metal atom is present or known as homonuclear complexes. And the complexes in which more than one metal atom is present are known as polynuclear complexes. All the examples which we are studying here are homonuclear complexes.

In the next coming post we will discuss about the rules for formula writing of mono nuclear coordination compounds and also for writing the names of coordination compounds with the study of different examples of coordination complexes.

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