What is Coordination Chemistry?

It is a part of chemistry, in which the study of compounds that have central atoms surrounded by molecules or ions, known as ligand, is known as coordination chemistry.

Basic Terms Used in Coordination Chemistry

Coordination entity

In this, the central atom is bonded to an appropriate number of ligands.

Example: [Ni (Co)₄]. 

Central metal atom

It is the metal ion which has empty d-orbitals to accept electron pairs from ligands and behaves as Lewis acid.

Ligands

They are directly bonded with the central metal ion in a complex compound.

Ambidentate ligands 

They are the ligands in which two donor atoms are present but only one atom is used to make coordination bond with metal.

Example: C≡N.

Coordination number 

It is the total number of sigma co-ordinate bonds attached to the central metal atom.

Example:             C.N.

    [Fe (CO)₅]        5

    [No (CO)₄]       4

Coordination and ionic sphere

Coordination sphere is a sphere in which the central metal ion is bonded with the donor atom.

Ions other than coordination sphere are known as ionic sphere. They are written outside the bracket.

Example:   [Ag(NH₃)₃]Cl

         [Ag(NH₃)₃] : Coordination sphere.

                 Cl: Ionic sphere.

Nomenclature of Coordination Compounds

1. First, write the name of the cation then the anion whether it is simple or complex. 
2. Before the name of metal ion, the name of ligands should be in alphabetical order. (This process is reversed from writing formula).
3. Name the anionic ligand then at the last use 'o' suffix for it.

Example: CN⁻: Cyano.

Cl⁻: Chloro.

• Name of the cationic ligand then we use 'ium'  suffix at the last and neutral ligand then we don't change anything in its name.

Example: NO⁺: Nitrosonium.

           H₂O: Aqua.

• If the ligands are more than one, we use di, tri, tetra, penta; but if the ligand has bi, tri, tetra, penta in its name then we use Bis, tris, tetrakis, pentakis for 2,3,4,5 numbers respectively.

Example: [CoCl(ONO)(en)₂]⁺ is chlorobis(ethylenediamine)nitritocobalt(|||) ion.

• At the last write the name of metal with its oxidation number in roman number.
• If the complex ion is cation, then metal is named same as the element.
• The neutral complex molecule is named similar to that of the complex cation.

Example: [Cr(NH₃)₃(H₂O)₃]Cl is called triamminetriaquachromium(III) chloride.

Isomerism in Coordination Compound

Isomers are the molecules in which the molecular formula is same but shows different type of structures due to different spatial arrangements and this phenomenon is known as isomerism. Because of the different arrangement of atoms, they differ in some physical or chemical properties. 

Types of Isomerism

Structural isomerism

• Linkage isomers: The compounds which have same molecular formula but differ in process of bonding of ligand to the central atom are called linkage isomers.

Example: [SCN]‾,  [Co(NH₃)₅(NCS–N)]²⁺ and [Co(NH₃)₅(SCN–S)]²⁺.

• Coordination isomers: It is shown by the compounds which have complex cation and complex anion. In this, the interchange of ligands between complex cation and complex anion takes place. 

Example: [CO(NH₃)₆], [Cr(CN)₆] and [Cr(NH₃)₆ ][Co(CN)₆].

• Ionisation isomers: In this, a ligand exchanges the place with the molecule which is outside the complex. The metal and the other left ligands are same. They give different ions in the solution.

Example: [Co(NH₃)₅Br], (SO₄) and [Co(NH₃) (SO₄)]Br.

• Hydration: In this, the water molecule interchanges place with ion outside the coordination sphere.

Example: [Cr(H₂O)₄Cl₂]Cl.2H₂Oand [Cr(H₂O)₅Cl]Cl₂.H₂O.

Stereoisomerism

• Geometrical isomerism: Geometrical isomers have identical empirical formula but differ in chemical and physical properties because of the different arrangement of ligands. They are usually easily separated by chemical or physical means. It is mostly formed by transition metals.

Main examples of this are of coordination numbers 4 and 6.

Example:                               C.N.

           1.  Pt [(NH₃)₂Cl₂                               4

           2. [Co (NH₃)₄Cl₂]⁺                           6

• Optical isomerism: It is shown by chiral molecules (molecules which do not have any element of symmetry). These isomers are non-superimposable mirror images of each other. They are optically active and they rotate the plane polarised light (PPL) in equal and opposite direction. The isomers which can rotate the PPL in clockwise direction are known as dextrorotators. The isomers which can rotate the PPL in anticlockwise direction are known as laevorotators.

Example: [PtCl₂(en)₂]²⁺.

Bonding of Coordination Compound

Werner Theory

Alfred Werner (considered as the father of coordination chemistry) proposed in 1893 that each metal in coordination compound shows 2 types of valences---Primary and secondary.

Example: In [Co(NH₃)₆]Cl₃

Cl‾: primary valency.

NH₃: secondary valency.

Werner’s isolated and studied varies cobalt ammines on the basis of his coordination theory in coordination chemistry.

VBT (Valence Bond Theory)

    It was given by Pauling.

• According to VBT, central atom of metal provides empty orbitals which are equal to its coordination number for the formation of covalent bonds with ligand orbitals.
• The empty orbitals of the metal hybridize to give an equal number of hybrid orbitals of equivalent energy.
• The metal ion can use ns, np, nd orbitals for hybridization. D-orbitals are involved in the hybridization that may be either inner or the outer d orbitals. The complexes formed in these two ways are known as low spin and high spin complexes respectively.
• Vacant hybrid orbitals of the metal ion overlap with the filled σ orbitals of the donar to form σ covalent bond represented as M L ←σ, the bond is also referred to as coordinate bond.     

Example: Octahedral complex [Co(NH₃)₆]⁺³.       

Limitations of VBT

• VBT doesn't give qualitative explanation of complex.
• Does not explain stability of complex & absorption spectra for compound.
• It does not explain magnetic properties.
• It does not give the reason of inner & outer or low spin & high spin complex.

Crystal Field Theory

• It is an electrostatic model.
• According to CFT "Metal atom and ligand generates an electric field because of their charge and electrons."

Following observations are made by the CFT:

• In this, central atom is considered as positively charged ion and ligand is considered as negatively charged ion.
• Ligand arranges itself towards the metal with its electron so and attractive for generates between ligand and metal atom.
• The ligands are considered as point charges which produces the electric field. This electric field changes the energy of d-orbitals of metal.
• Due to this electric field of ligand the degeneration of d orbital of metal splits into two parts in group of 2 and 3 orbitals.
• These orbitals are known as e_g and t_2g.
• The energy of these orbitals depends on the type of coordination complexes.
• There is no interaction between the metal orbital and ligands orbital.
• According to CFT, ligands are arranged in a series according to their strength.

Importance of Coordination Compound

• In Medicine:Cisplatin is used for the treatment of cancer.
• In Photography:Developing of the film involves complex formation.
• Rate of hardness of water is determined with the help of titration with Na₂EDTA.The Ca²⁺ and Mg²⁺ ions form stable complexes with EDTA.
• They are very important for biological systems such as

Hemoglobin in blood = complex of iron. Chlorophyll in plants = complex of Mg²⁺.

Common Mistakes

1. Stability of coordination compounds is not explained.
2. Color in coordination compounds due to d-d transition.

Context and applications

Coordination chemistry is significant in the professional exam both graduate and post graduate in chemistry especially for Bachelors and Masters in Chemistry.

Related Concepts

• Organometallic chemistry.
• Coordination polymer chemistry.
• Molecular orbital theory.