Coordination Compounds are those compounds which Retain their identities even when dissolved in water or any other solvent and their properties are completely different from those of constituents.
•Complex ion may be defined as an electrically charged ion which consists of central metal atom or ion surrounded by a group of ions or neutral molecules. Example: [Ni(NH3)6]2+.

Double Salts: are addition or molecular compounds which are formed by two apparently saturated compounds but they lose their identity when dissolved in water.

Examples : Mohr’s salt: FeSO4.(NH4)2SO4.6H2O

Potash alum : K2SO4.Al2(SO4)3.24H2O

When Mohr’s salt dissolved in water it gives charateristic properties of Fe2+ ,NH4+, SO42- ions.thus double salts are stable in solid state but break up into constituents when dissolved in water. But in case of coordination compounds such as [Fe(CN)6]4- ,complex ion does not dissociate into Fe2+ & CN Ions when dissolved.

Central ion and ligand

The cation to which one or more neutral molecules or ions are coordinated is called central ion while the molecules or ions attached to the central ion are called ligands.

Coordination Number:

The total number of ligands attached to the central ion is called the coordination number.

Coordination Sphere:

The molecules or ions bonded directly to the central metal ion constitute coordination sphere.

Charge on a complex ion

The charge carried by s complex ion is the algebraic sum of charges carried by the central ion and the ligands coordinated to it.

For example, [Ni(NH3)6]2+ carries a charge of +2 because Ni2+ ion carries a charge of +2 and ammonia molecule is neutral.

Types of ligands

Unidentate or monodentate ligand

Ligands which donates only one pair of electrons and can coordinate to the central ion through one donor atom.

Examples : F ,Cl,Br,CN,OH,NH3,H2O,CO,Py etc.

Bidentate ligands

Ligands which have two donor atom ,ie have tendency to coordinate central Ion through two donor atoms are called Bidentate ligands.

Example: oxalate ion , Ethylenediamine ,Dimethyl glyoximate ion.

Polydentate ligands:Ligands having more than two donor atoms present in the molecule. Example EDTA.

Ambident ligand: the monodentate ligand which can coordinate with the central atom through more than one coordination atom in their molecules. Example CN can coordinate through C or N similarly , SCN can coordinate through S or N.

Coordination polyhedron: is the spatial arrangement of the ligand atoms which are directly attached to the central atom. For example ,[Co(NH3)6]3+ has octahedral geometry, [PtCl4]2- has square planar geometry and [Ni(CO)4] has tetrahedral geometry.

Werner’s coordination Theory(postulates)

a) metals exhibit two types of valancy primary & secondary.

Primary-Oxidation number(ionizable)

Secondary-Coordination number(non-ionizable)

b)Every metal has fixed number of coordination number.

c)Primary valency are non-directional whereas Secondary valency is directional & give the Structure of the Complex ion and determine the Stereochemistry of the complex.

d)Metal atom tends to satisfy both its primary as well as secondary valancies.Primary valancies are satisfied by negative ions whereas secondary valancies are satisfied either by negative ions or by neutral molecules. In certain cases ,a negative ion may satisfy both types of valancies.

Experimental Verification of Werner’s Theory

1.Precipitation data: the number of ions furnished by a complex in a solution can be determined by precipitation reactions.For example ,number of Cl ion in a solution can be determined by the treatment of AgNO3 solution ,and calculating the amount of white precipitate of AgCl formed per mole of compound.Example:

CoCl3.6NH3 + AgNO3 ——> AgCl (corresponding to 3Cl ions )

CoCl3.5NH3 + AgNO3 ——> AgCl (corresponding to 2Cl ions )

Which supports Werner’s Theory
[Co(NH3)6]Cl3 , [Co(NH3)5Cl]Cl2

2.Conductance Measurements: The measurement of molar conductance (Λm) of solution of coordination compounds help to estimate the number of ion furnished by the compound in Solution. By comparing the ∆m of the compound with those of some known electrolytes , Werner was able to predict the number of ions present in the solution.

3.Number of isomers Formed: Werner attempted to assign structures of different coordination compounds by comparison of the number of known isomers and the number of theoretically possible structures.


are the cyclic (or ring) compounds that are formed when a ligand binds the central metal atom or ion through two or more atoms of a ligand.

•Chelated Complexes are more Stable then the Non-chelated Complexes.

•EDTA is used to form chelated ion with metal because these ions are insoluble in water which can be easily filtered.

Factors affecting Stability of chelates

a) Chelates Effect :- chelating ligands form stable complexes than ordinary ligands .this enhances the stability of complexes containing chelating ligands is called chelate effect.

b) As number of chelating ring increases , Stability of complex increases.

c)Size of chelating ring: 5 membered chelates are most stable.

d)Chelating ligands which do not contain unsaturated groups(double bonds) form five membered stable rings. And chelating ligands containing unsaturated groups(double bonds) form 6 membered stable rings.

e) Due to stearic hinderance ,ligands with larger group form unstable compounds as compared to the analogous ligands containing smaller groups.

We’ll discuss more about stability of chelates in my coming posts.

Homolyptic & Hetrolyptic Complexes

•Complexes in which the metal is bound to only one kind of donor group are called homoleptic complexes. Eg: [Ni(NH3)6]3+

•Complexes in which the metal is bound to more than one kind of donor groups are called Hetrolyptic complexes. Eg: [NiCl2(H2O)4].

Nomenclature of Coordination Compounds

1.Order of Naming ions: in ionic complexes cations are named first and then the anion.

•Non-ionic complexes are given one word name.

2. Ligands are Named first and then the central metal ion is named.

3.Naming of Ligands: names of anionic ligands end in


,positive ligands end in


and neutral ligands are named as such.

Negative Ligands
F fluorido
Cl chlorido
Br bromido
OH hydroxo
CN cyano
CH3COO acetato
NO2 nitro
ONO nitrito
SO42- Sulphato
C6H5 phenyl
SCN thiocynato
NCS isothiocyanato
CO32- Carbonato
NH2- imido
NH2 amido
H hydrido
S2- thio
HS mercapto
O2- oxo
O22- peroxo
N3 azido
CH3O methoxo
C5H5 cyclopentadienyl
Positive ligands
NO+ nitrosonium
NH2NH3+ hydrazinium
NO2+ nitronium
Neutral ligands
NH2CH2CH2NH2 ethylenediamine (en)
C6H5N pyridine (py)
(C6H5)3P triphenylphosphine
CO carbonyl
NO nitrosyl
NH3 ammine
H2O aqua

4.Ligands are named in alphabetical order of preference without any consideration of charge.

5.when more than one ligands if particular kind are present in a complex , the prefixes di-,tri- ,tetra-, Penta- ,hexa- etc. Are used to indicate their number.

•when name of ligand includes numerical prefix (di ,tri ,tetra ) then prifixes bis ,tris,tetrakis are used for two ,three and four ligands resoectively.

6.when the complex is anionic ,the name of the central metal atom ends in -ate .

Such as ferrate for Fe, cuperate for Cu , argentate for Ag , aurate for Au ,Stannate for Sn etc.

However ,if complex is cationic or neutral the name of metal is given as such.

7.Oxidation State of central metal ion is designated by Roman numeral in the brackets at the end of the name of complex without a gap.

8.Point of attachment:when ligands can coordinate through more than one atom ,then we use different names for alternative mode of attachment.
For example:- SCN (through S)is named as “thiocynato” while
NCS(through N)is named as “isothiocyanato”.

9.Geometrical isomers : Geometrical isomers are named by the use of the term CIS to designate adjacent sides and trans for opposite sides.

10.Optical isomers: optically active compounds are designated by the symbols (+) or d for dextrorotatory and (-) or l- for laevorotatory.

11.Bridging Groups:- for ligands which can act as bridge between two metal atoms , μ is written before their names.


a)K[Pt(NH3)Cl5] – potassiumamminepentachloroplatinate(iv)

•here complex is anionic i.e metal ends with -ate, oxidation state of Pt is +4, ligands are arranged alphabetically and Penta is used for 5 Cl ligands.

b)[Co(NH3)5Cl]Cl2 – pentaamminechlorocobalt(iii)chloride

•here complex is cation and chloride is anion, oxidation state of Co is +3 .we write pentaammine for 5 NH3 ligands ,where “a” is not dropped.

c)[(NH3)5Cr-OH-Cr(NH3)5]Cl5 ,- μ-hydroxobis(pentaamminechromium(iii)]chloride.

d) bridging coordination compound tetraamminecobalt(iii)-μ-amido-μ-nitrotetraamminecobalt(iii)nitrate

*If you have any doubt regarding nomenclature or naming of a perticular complex then leave your questions in comment box.

Isomerism in Coordination Compounds

(A)Structrual Isomerism

1)Ionisation Isomerism:Ionisation isomers are the compounds which have same molecular formula but give different ions in Solution.

Example: [Pt(NH3)4Cl2]Br2 , [Pt(NH3)4Br2]Cl2

2)Hydrate Isomerism: Arises from the Replacement of a Coordinated group by a water of hydration.

Example: [Cr(H2O)6]Cl3 ,[Cr(H2O)5Cl]Cl2.H2O.

3.Coordination Isomerism : This Isomerism occours in compounds containing both cationic and anionic complexes and occurs because of different distribution of the ligands.

Example: [Cu(NH3)4][PtCl4] , [Pt(NH3)4][CuCl4]

4.Linkage Isomerism: this type of Isomerism is exhibited by complexes containing monodentate ligands having more than one donor atom.

Example: [Co(NH3)5(NO2)]Cl2 (yellow) ,[Co(NH3)5(ONO)]Cl2 (Red)

5.Coordination Position Isomerism: This type of Isomerism is exhibited by bridging Complexes and Results from different placement of ligands.

Example : coordination position isomers compound

6.Polymerisation Isomerism: this is not a true Isomerism. it is used to donate compounds having the Same Empirical Formula but different molecular formula.

Example: [Co(NH3)3(NO2)3] & [Co(NH3)6][Co(NO2)6]

(B) StereoIsomers : Differ in Spatial Arrangement.

1.Geometrical Isomerism:

Due to ligands Occupying different position around the Central ion.

Coordination Number-4 (Square planner)

•1-3 ,2-4 Are Trans and other are Cis. Geometrical Isomerism

a)MA2BC geometrical Isomerism square planer compound

b)MABCD :Shows three isomers, Structure of these can be written by fixing the position of one ligand.

geometrical Isomerism of MABCD square planer


geometrical Isomerism of M(AB)2 square planer

Bridged binuclear Complexes M2A2X4

geometrical Isomerism bridged binuclear complex M2A2X4

Coordination number 6: Octahedral

a)[MA4B2] ,[MA4BC] shows Geometrical isomers.

•1-6 ,2-4 ,3-5 are trans while the others are CIS
geometrical Isomerism octahedral

b)Complexes having bidentate Ligands of the type [M(AA)2B2] or [M(AA)2BC]

Example:geometrical Isomerism of bidentate ligands

c)[MA3B3] :- When the three ligands are on the Same triangular face of the octahedron , the isomer is called Facial or Face isomer.

•when the three ligands are on the same equatorial plane of the octahedron are called meridonal or mer-isomer.ligands
face and mer isomers

d)[M(ABCCDEF)] :it exhibit 15 isomers by keeping position of one ligand constant and rotating other. Example :-[Pt(Py)(NH3)(NO2)(Cl)(Br)(I)] is the only compound of such type.

Optical Isomerism

The Isomers which rotate the plane of polarised light equally but in opposite direction are called optically active isomers.

Dextrorotatory : Isomer which rotate the plane of polarised light to the right is called dextrorotatory(d).

Laevorotatory:Isomer which rotate the plane of polarised light to the Left is called Laevorotatory(l).

•Substance Should not have a palne of Symmetry in its Structure and Extent of Rotation of plane polarised light by the two isomers is exactly same but in opposite direction.

Racemic mixture

Solution containing equal Concentration of two isomers the rotation by two isomers cancel each other.Hence there is no rotation of plane of polarised light.

Optical Isomerism : C.N.-4 Tetrahedral

Tetrahedral Complexes are expected to exhibit optical Isomerism. Bcoz there is no plane of symmetry in their molecule. But it is not possible to isolate optically active Compounds of tetrahedral Complexes when all the four groups are different.

•Because these Complexes are labile and it is not possible to isolate them.

¶ Tetrahedral Complexes Containing Unsymmetrical bidentate ligands have been Resolved into optically active form.

•Square Planer Complexes are not optically active.

Optical Isomerism: C.N.-6 Octahedral

All these forms shows optical Isomerism : [M(AA)3] , [M(AA)2B2] , [M(AA)2BC] , [M(AA)B2C2] .

a) [M(AA)3] ,example: [Co(en)3]3+

Optical Isomerism of [Co(en)3]3+

b)[M(AA)2B2] ,example [Co(en)2Cl2]+

optical Isomerism of [Co(en)2(Cl)2]+

•Trans form has plane of symmetry hence there are only 3-isomers.

c) [M(AA)B2C2] , example : [Co(en)(NH3)2(Cl)2]+

optical Isomerism of [Co(en)(NH3)2(Cl)2]+

d)Complex Containing hexadentate ligands ,example: [Co( EDTA)]

optical Isomerism of [Co(EDTA)]-

Resolution of Racemic mixtures

The process of converting d or l form of an optically active isomer into Racemic form (dl) is called Racemisation.

•on the other hand , the process of separation of Racemic mixture into d or l form ( enantiomers) is called resolution.

Methods of resolution

1.Chemical methods : Selective crystallization of diastereomers

The diastereomers obtained when the Racemic mixture of the complex cation or anion is allowed to react with an optically active anion or cation respectively. So that these can be separated by methods such as fractional crystallisation ,precipitation ,extraction or adsorption etc.

2.Selective adsorption on optically adsorbent.

3.Stereospecific Reactions

4.Mechanical Seperation of Racemic mixtures.

Applications of Coordination Compounds

1.Estimation of hardness of water: it is estimated by simple titration with EDTA .as Ca2+ and Mg2+ ions form Stable Complexes with EDTA.

2.In Electroplating and water treatment.

3.In dyeing , Catalysis & medicines.

4.Biological importance

Chlorophyll is Complex of Mg2+

Hemoglobin is Complex of Fe2+

Vitamin B12 is Complex of Co3+

*KCN is Toxic where as K4[Fe(CN)6] is non-toxic ?

Because CN is Toxic in nature and there is no free CN ion in K4[Fe(CN)6].

K4[Fe(CN)6] ——-> [Fe(CN)6]4- + 4K+