Coordination Compounds class 12 notes Chemistry chapter 9

Coordination Compounds class 12 notes Chemistry chapter 9

This article is on the Coordination Compounds Class 12 Notes of Chemistry. The notes on coordination compounds of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the Coordination Compounds chapter of Class 12 Chemistry for the students so that they can give a quick glance of the chapter during exams.

Introduction to Coordination Compounds, Nomenclature and Isomerism:

Coordination compound or Complex compounds are those compounds which contain a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules. There is a coordinate bond between the central metal atom and these ions or molecules, e.g., K4 [Fe(CN)6 ].

In other words, these compounds contain complex ions. Complex ions are the ions with a central metal ion bonded to one or more molecules or ions e.g.,     [Cu (NH3)]4+.

The salt of the complex ions is called complex compounds or coordination compounds.

Double Salts:

These are the salts which are formed by the combination of two or more stable compounds in a stoichiometric ratio. Double salt like Mohr’s salt, FeSO4.(NH4 )2 SO4.6H20 dissociates into simple ions completely when dissolved in water but complex compound such as K4 [Fe(CN)6 ] does not dissociate into its ions (i.e., K+, Fe2+ and CN ions).

[ Fe(CN)6]4-


4  —> Charge

[ Fe(CN)6] —->  Coordination sphere

6 —-> Coordination number

CN —-> Ligand (Lewis base)

Fe —-> Central metal

Werner’s Theory of Coordination Compounds:

Alfred Werner was the one who defined the ideas about the structures of coordination compounds. According to Werner’s theory of coordination compounds, metal possesses two types of valencies, called primary or ionisable valency (oxidation number) and secondary or non-ionisable valency (coordination number).

Main postulates of Werner theory are

(i) In coordination compounds, a metal shows two types of valency (linkage) i.e., primary valency and secondary valency.

(ii) Primary valencies are satisfied by anions. They are normally ionisable and non-directional.

(iii) The secondary valencies are satisfied by neutral molecular negative ions. They are non-ionisable and directional. It represents the coordination number and is fixed for a metal.

e.g., the structure of [CoCl2(NH3)4]+Cl can be represented as showing primary and secondary valency as

NH3 — Primary valency

Cl  — Secondary valency

Definitions of Some Important Terms Pertaining Coordination Compounds:

  1. Coordination Entity:

Coordination entity is an electrically charged species which constitutes a central metal atom or ion attached to a fixed number of ions or molecules. It may be positive, negative or neutral. e.g., [CoCl3 (NH3) ]2.

  1. Central Metal Atom or Ion:

In a coordination entity, the metal atoms or ions with which a definite number of ligands are attached in a definite geometry are called central metal atom or ion. e.g., Fe2+, Cu3+, Ni, Pt2+ are central ions which are also known as Lewis acids.

  1. Ligands:

Molecules or ions which donate a lone pair of electron to the central atom or ion are called ligands. There are the ions or molecules bounds to the central atom/ ion in the coordination entity. It is a Lewis base.

Ligands are classified into three categories on the basis of the charge

(i) Neutral ligands e.g., H2O, CO, NH3, NO

(ii) Negative ligands e.g., F, CI, Br, I

(iii) Positive ligands e.g., NO+, NO2, NH2 –NH3+

On the basis of denticity (donor sites) ligands are classified as follows

Unidentate: When a ligand is bonded with one donor site to the central metal atom or ion e.g., Cl, CO, NH3, H2O etc.

Didentate or Bidentate: When a ligand is bonded with two donor site to the central metal atom or ion, e.g., 1, 2-ethane diamine (NH2 —CH2 —CH2 —NH2).

Polydentate: The ligands which contain three or more donor sites to the central metal atom or ion, e.g., ethylenediamine tetraacetate ion (EDTA). It is an important hexadentate ligand.

Ambidentate: When a ligand can ligate through two different atoms is known as an ambidentate ligand. e.g., —NO2, SCN, ONO.

Chelating Rings: When a di- or polydentate ligand is bonded through two or more donor sites to a single metal ion is called a chelate ligand. The number of such ligating groups is called the denticity.

  1. Coordination Number (CN):

The total number of ligands to which the metal is directly attached is called the coordination number.

[Co(NH3)6]3+, CN = 6

[Ni(CO)4], CN =. 4

[Fe(C204)3]3-, CN = 3 x 2 = 6   (since C204 is bidentate)

  1. Ionisation Sphere:

The part of the coordination compound which is present outside the square brackets is known as ionisation sphere. e.g., in [Cu(NH3)4] SO4, SO42- constitutes ionisation spheres.

  1. Coordination Sphere:

The central atom or ion and the ligands present enclosed in the square brackets are collectively known as the coordination sphere. The ionisable group is written outside the bracket and called counter ions. e.g., in K4 [Fe(CN)6], the coordination sphere is [Fe(CN)6]4-

  1. Coordination Polyhedron:

It is the spatial arrangement of the ligand atoms which are directly attached to the central atom or ion. The most common coordination polyhedral is octahedral, square planar and tetrahedral. E.g., [Co(NH3)6]3+ is octahedral, [Ni(CO)4] is tetrahedral and [PtCl4]2- is square planar.

  1. Oxidation Number of Central Atom:

It is defined as the charge it would carry when all the ligands are removed along with electron pairs which are shared with the central atom.

Homoleptic Complexes are bounded to only one type of donor groups e.g., [Co(NH3)6]3-. Heteroleptic Complexes are bounded to more than one kind of donor groups, e.g., [Co(NH3)4Cl2]+

Naming of Mononuclear Coordination Compounds:

 The following rules are used while naming coordination compounds

  1. Cationic Complex:

(i) The ligands are named alphabetically and prefixes like mono, di, etc are used to designate the no. of the ligands.

(ii) The name of the central atom and the oxidation state of the metal is designated by Roman No. in the bracket.

(iii) Name of the ionisable metal

  1. Anionic Complex:

(i) Name of the ionisable metal

(ii) The ligands are named alphabetically and prefixes like mono, di, etc are used to designate the no. of the ligands.

(iii) The name of the central atom and the oxidation state of the metal is designated by Roman No. in the bracket.

  1. Neutral Complex:

(i) The ligands are named alphabetically and prefixes like mono, di, etc are used to designate the no. of the ligands.

(ii) The name of the central atom and the oxidation state of the metal is designated by Roman No. in the bracket.

Isomerism of Coordination Compounds:

When two or more compounds have the same molecular formula but different arrangements of atoms are said to exhibit isomerism. They differ in one or more physical or chemical properties.

Two major types of isomerism of coordination compounds are

  1. Structural Isomerism:

(i) Ionisation Isomerism: When different ions are produced on ionisation. e.g., [Pt(NH3)4(OH2)]SO4 and [Pt(NH3)4(SO4)](OH2).

(ii) Coordination Isomerism: When cation and anion both are complexes.

e.g., [Cr(NH3)6] [Co(C204)3 and [Co(NH3)6] [Cr(C204)3].

(iii) Hydrate (solvate) Isomerism: When the number of water (solvent) molecules are different

e.g., [Cr(H20)6]Cl3 and [Cr(H20)5Cl]Cl2. H2O.

(iv) Linkage Isomerism: When an ambidentate ligand is present.

e.g., [Co(NH3)2 (NO2)2 (py)2] NO3 and [Co(NH3)2 (ONO)2 (py)2] NO3 .


(i) Geometrical Isomerism:

It is shown by complexes of the type square planar complex, [MA2X2], [MABX2], octahedral complex, [MA2X4], [MABX4 ], [MA4X2] and [MA3X3]. [MA3X3] shows fac and mer geometrical isomerism.

Square planar complex of the type MABXL shows three geometrical isomers: two cis and one trans.

(ii) Optical Isomerism:

It is shown by complexes of the type [M(AA)3], [M(AA)2B2] (only cis form, not trans-form), [M(AA)2AB] (only cis form, not trans-form), [M(AA)A2B2] and [M(EDTA)] etc.

where, (AA) = bidentate ligand and EDTA = hexadentate ligand.

Optical isomers that cannot be superimposed on one another, are called enantiomers. The molecules or ions that cannot be superimposed are called chiral and thee two forms of it are dextro (d) and laevo (l).

Octahedral complexes show both geometrical as well as optical isomerism. Tetrahedral complexes do not show geometrical isomerism because the relative positions of unidentate ligands attached to central metal atom are the same with respect to each other.

Bonding in Coordination Compound:  VBT and CFT

Many approaches have been put forth to explain the nature of bonding in coordination compounds, Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT).

Valence Bond Theory (VBT):

(i)If coordination number is four, the complex is tetrahedral (sp3) usually in case of weak field ligand and square planar (dsp2) in case of strong field ligand.

(ii) If the coordination number is 6, the complex is octahedral with sp3d2 or d2sp3 hybridisation.

(iii) A strong field ligand forces the d-electrons of central metal to pair up against Hund’s rule. The strong field ligand which causes this are CO, NO, CN NO2, NH3, en (ethylenediamine). Some weak field ligands are H2O, X , NO3, ROH, etc.

(iv) If complex formed involves inner (n-1)d orbitals for hybridisation, it is called inner orbital or low spin or spin paired complex. e.g., [Ni(CN)4 ], whereas if complex formed involves, outer (n) d-orbitals for hybridisation, it is called outer orbital, high spin or spin free complex. e.g., [Ni(H20)6 ]2-.

 (v) The complex may be diamagnetic (due to the absence of unpaired electrons) or paramagnetic (due to the presence of unpaired electrons).

Limitations of Valence Bond Theory (VBT):

(i) A number of assumption is involved.

(ii) Quantitative interpretation of magnetic data is not given.

(iii) No explaination of the colour exhibited by coordination compounds.

(iv) Exact predictions regarding the tetrahedral and square planar structures of 4 coordinate complexes cannot be made.

(v) Distinguish between weak and strong ligands cannot be made.

(vi) A quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds is not given.

Crystal Field Theory (CFT):

It is a more appropriate theory than VBT. According to CFT, under the influence of the ligand field, the degeneracy of the d-orbital is destroyed and it splits into two or more energy levels. The extent of splitting depends upon the strength of ligand. A strong ligand causes greater splitting while a weak ligand causes smaller splitting.

Crystal Field Splitting in Octahedral Coordination Entities:

The difference of energy between the two sets of d-orbital (in octahedral complexes) is called Crystal Field Splitting Energy (CFSE) or ∆0. The magnitude of ∆0 depends upon nature the ligands.

The increasing order of ∆0 is given below

I< Br < S2-  < SCN  < Cl < F< OH< C2042- < 02- <H20  < NCS- < NH3 < en < NO2 < CN < CO

(i) If ∆0 <P, the fourth electron enters one of the eg orbitals giving the configuration t2g3 eg1. Ligands for which ∆0 <P, are weak field ligands and they form high spin complexes.

(ii) If ∆0 > P, the fourth electron enters t2g orbital giving the configuration t2g4 eg0 . Ligands for which ∆0 >P, are strong field ligands and they form low spin complexes.

Coordination Compounds class 12 notes Chemistry chapter 9

Colour in Coordination Compounds:

When the light of certain frequency falls on the complex, it absorbs light from the visible range for the transition of electrons from lower to higher level. Colour of the compound is the complementary colour of the absorbed light. This is called d-d transition.

Limitations of CFT:

(i) The anionic ligands should exert the greatest splitting effect as the ligands are point charges. The anionic ligands are actually found at the lower end of the spectrochemical series.

(ii) CFT does not take into account the covalent character of bonding between the ligand and the central atom and treats the metal-ligand bond as purely ionic.

Important Properties of Coordination Compounds:

Bonding in Metal Carbonyls:

(i) The metal-carbon bond in metal carbonyls possesses both a and n characters. The bond between CO and the metal is strengthened due to the synergic effect which is created by the metal to ligand bonding.

(ii) The oxidation state of metal in a metal carbonyls is zero.

(iii) Bonding in metal carbonyl can be diagrammatically understood as

Stability of Complex:

The stability of complex refers to the extent up to which it exists in a solution as the coordination sphere.

Factors Affecting Stability of Complex:

(i) Charge on the Central Metal Ion: Higher the charge on the central metal ion, greater is the stability of the complex.

(ii) Nature of the Metal Ion: Groups 3 and 6 and inner transition elements form stable complexes when donor atoms of the ligands are N, 0 and F. The elements after group 6 of the transition metals form stable complexes when the donor atoms of the ligands are the heavier members of N, 0 and F family.

(iii) Basic Nature of the Ligands: Greater the basic strength, greater is the stability of the complex.

(iv) Presence of Chelate Rings: Its presence increases the stability of the complex. It is called the chelate effect. It is maximum for the 5 and 6 membered rings.

(v) Effect of Multidentate Cyclic Ligands: If the ligands are π multidentate and cyclic without any steric effect, the stability of the complex is further increased.

Stability Constant:

The relative stabilities of coordination complexes can be compared in terms of stability constant (k), also denoted by β.

For a reaction,    mn+ + nLx-     —>   [MLn] b+   ;k =  [MLn] b+ / [Mn+][ Lx-]n

Importance and Applications of Coordination Compounds:

(i) Extraction of metals e.g., Au and Ag are extracted by dissolving it in NaCN. Purification of metals e.g., Ni.

(ii) Estimation of the hardness of water.

(iii) In qualitative and quantitative chemical analysis.

(iv) In medicine e.g., cis-platin [PtCl2 (NH3)2 I is used in the treatment of cancer.

(v) Electroplating of metals involves the complex salts as an electrolyte.

(vi) Haemoglobin is a complex of iron, vitamin. B12 is a complex of cobalt, chlorophyll is a complex of magnesium.

(vii) It is used in homogeneous and heterogeneous catalysis. e.g., Wilkinson catalyst is used for hydrogenation of alkenes.

(viii) It is used as an antiknock [(C2H5)4 Pb].

Roles of Coordination Chemistry:

1) Role of Coordination Compounds in Biological Systems:

 (a) Haemoglobin, the oxygen carrier in blood, is a complex of Fe2+ with porphyrin.

(b) The pigment chlorophyll in plants, responsible for photosynthesis, is a complex of Mg2+ with porphyrin.

(c) Vitamin B12 is a complex of cobalt.

 (ii) Role of Coordination Compounds in Medicinal Chemistry:

(a) The platinum complex cis-[Pt(NH3)2 Cl2 ] (cis-platin) is used in the treatment of cancer.

(b) EDTA complex of calcium is used in the treatment of lead poisoning.

(c) The excess of copper and iron present in the animal system are removed by the chelating ligands. D-penicillamine and desferrioxime B- via the formation of complexes.

(iii) Role of Coordination Compounds in Analytical Chemistry:

(a) Detection of Cu2+

(b) Ni2+ is detected by the formation of a red complex with dimethylglyoxime (DMG).

(c) The separation of Ag+ and Hg2+ in group I is based on the fact that AgCl dissolves NH3, while Hg2Cl2 makes an insoluble black substance with it.

(iv) Role of Coordination Compounds in the Extraction/Metallurgy of Metals:

(a) Extraction of various metals from their ore involves complex formation. e.g., silver and gold are extracted from their ore by forming a cyanide complex.

(b) Mond process for the purification of nickel.


This article has basically highlighted the chemistry of coordination compounds in the form of notes for class 12 students in order to understand the basic concepts of the chapter. The notes on coordination compounds have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.

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