Coordination Compounds | Unit 9 | Chemistry


The compound which have a core atom (or cation) that is coordinated to an appropriate number of anions or neutral molecules. These compounds often maintain their identity both in solution and in the solid state. These could be [ Co(NH3)6 ] 3+ , [ NiCl4 ]2- ,  [ Ni(CO)4 ] or neutral species with either a positive or negative charge.

Werner Theory

Werner proposed the following idea in 1893 to account for the structure and bonding of coordination compounds.

Metals exhibit two different types of valences in coordination compounds: Primary Valency and Secondary Valency.

The ionizability of primary valency.

Secondary valency cannot ionize, according to this theory only partially succeeded in explaining several characteristics of coordination molecules.

Differences between Complex Ions and Double Salts

When dissolved in water, double salts entirely separate into simple ions, whereas complex ions do not.

Unlike in complexes, where the negative ions or molecules surrounding the core metal atom are outside of its normal valency, the metal atom or ion in double salts exhibits the typical valency.

Some Important Terminology for Coordinating Compounds
Coordination Polyhedral

Such spatial arrangements are known as coordination polyhedral in contemporary formulations.

The species outside the square brackets are referred to be counter ions, and the species inside them are coordination entities or complexes.


Compounds known as isomers have the same chemical formula but different structural configurations.

The isomerism categories that coordination compounds exhibit are:

(a) Geometrical isomerism:

Geometrical isomerism  also known as cis-trans isomerism.

Cis-trans isomerism

Two coordination compounds are considered to be geometrical isomers if their ligand arrangements differ. The isomer is known as “cis-form” when two identical ligands are positioned adjacent to one another, and as “trans-form” when they are arranged opposite to one another.

(b) Optical isomerism:

The chiral molecule, or those molecules without a plane of symmetry, such as [ Cr(ox)3 ], exhibits optical isomerism.

(c) Linkage Isomerism:

When an ambidentate ligand, such as [ CO(NH3)5N02 ] 2+ and [ Co(NH3)(-ONO) ] 2+, is present in the coordination sphere, linkage isomerism develops in complexes.

(d) Coordination Isomerism:

Due to the exchange of ligands between the cation and anion entities, coordination isomerism occurs in complexes consisting of cationic and anionic coordination entities, such as [ CO(NH3)6 ] [ Cr(CN)6 ] and [ Co(CN)6 ] [ Cr(NH3)6 ].

(e) Ionization Isomerism:

Ionization isomerism results from the exchange of ions between the metal ion’s coordination sphere and the ions outside of the sphere. In aqueous solution, these two isomers produce distinct ions, such as [Co(NH3)5 Br ] 2+ SO4 2- and [ Co(NH3)5 (SO4) ]+ Br

(f) Solvate Isomerism

When water is either a component of the coordination entity or is present outside of it, such as in the case of CrCl3-6H2O, solvate or hydrate isomerism occurs.
Magnetic properties:

(1) Inner Orbital (Low Spin) Complexes

Such as [Fe(CN)6 ]4-, [CO(NH3)6 ]3+, [Cr(NH3)6]3+, [Fe(CN)6]2+, [Fe(H2O)6]2+, [(MnCN)6]3-, etc., are complexes in which metal hybrid orbitals are produced by hybridization of (n-1) d,

(2) Outer Orbital (High Spin) Complexes

They are those compounds, such as [MnF6] 3-, [FeF6] 3-, [ Ni(NH3)6 ] 2+, [Ni (H2O)6] 2+, etc., in which the metal’s hybrid orbitals are created by hybridizing ns, np, and nd-vacant orbitals.

Crystal Field theory

The ligands are assumed to be point charges in the crystal field theory.
The point charges interaction with the core metal’s electrons is electrostatic in nature.
In an isolated gaseous metal ion, the 5d-orbitals have the same energy, making them degenerate.

Stability constant

A coordination compound’s stability is determined by its stability constant.

For response generally,

READ MORE:  Chemistry Class 12 Chapter 8 – The d-and f-Block Elements

Drawbacks of Crystal Field Splitting

Given that ligands are assumed to be point charges, anionic ligands should have the largest effect on splitting. The low end of the spectrochemical series is where anionic ligands are actually found.

The covalent nature of the connection between the ligand and the central atom is not taken into account.

Metal Carbonyl Bonding

The metal carbon (M-C) bond in metal carbonyl has the properties of both a – and -bond.

Coordination compound’s significance and applications include:

(a) Several quantitative and qualitative chemical analyses.

(b) During the extraction of metals like gold and silver.

(c) The synthesis and subsequent disintegration of coordination compounds can be used to purify metals like Ni.

(d) In biological systems, chlorophyll, a coordination complex of magnesium, is the pigment responsible for photosynthesis. An oxygen transporter is hemoglobin, a coordination molecule of iron.

Chelate treatment in medicinal chemistry

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