Double salts are formed when two salts crystallize together in a stoichiometric ratio from their saturated solution. They dissociate into simple ions when dissolved in water. Coordination compounds retain their identity in both the solid and dissolved states. The metal acts as a Lewis acid and provides empty orbitals to accept electrons from ligands, which act as Lewis bases by donating electron pairs. Coordination compounds have defined geometries depending on the metal's hybridization and the ligand environment. Crystal field theory and valence bond theory are used to explain properties like color and magnetism.

1. JustForChemistry
Double salt-
a) When two salts in stoichiometric ratio are crystallized together from their saturated
solution they are called double salts
b) Example- FeSO4.(NH4)2SO4.6H2O (Mohr’s salt), K2SO4Al2(SO4)3.24H2O (potash alum)
c) They dissociate into simple ions when dissolved in water.
Coordination compounds- Addition compounds which retain their identity in dissolved state also like solid state. In
coordination compounds the atoms or group of atoms have been attached to central metal atom or ion beyond the number
possible according to electrovalent or covalent bonding.
Metal- act as lewis acid, provide empty orbitals to accept electrons
Ligand- act as lewis base an ion or molecule capable of donating a pair of electrons to the central atom via a donor atom. ) It
may be neutral, positively or negatively charged.
• Coordination number-
a) The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the
metal is directly bonded.
b) Example: In the complex K4[Fe(CN)6], the coordination number of Fe is 6.
• Coordination sphere-
a) The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the
coordination sphere.
b) Example: In the complex K4[Fe(CN)6], [Fe(CN)6]
4-
is the coordination sphere.
• Counter ions:
a) The ions present outside the coordination sphere are called counter ions.
b) Example: In the complex K4[Fe(CN)6], K
+
is the counter ion.
• Coordination polyhedron:
a) The spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion defines a coordination
polyhedron about the central atom.
b) The most common coordination polyhedra are octahedral, square planar and tetrahedral.
c) Examples: [PtCl4]
2-
is square planar, Ni(CO)4 is tetrahedral while [Cu(NH3)6]
3+
is octahedral.
• Charge on the complex ion:
The charge on the complex ion is equal to the algebraic sum of the charges on all the ligands coordinated to the central metal
ion
• Oxidation number of central atom:
The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed
along with the electron pairs that are shared with the central atom.
• Homoleptic complexes:
Those complexes in which metal or ion is coordinate bonded to only one kind of donor atoms. For example: [Co(NH3)6]
3+
• Heteroleptic complexes:
Those complexes in which metal or ion is coordinate bonded to more than one kind of donor atoms. For example:
[CoCl2(NH3)4]+, [Co(NH3)5Br]2+
Dentacity- No. of ligand donor atoms that attach to the metal.
Unidentate ligands: Ligands with only one donor atom, e.g. NH3, Cl
-
, F
-
etc.
Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine (en), C2O4
2-
oxalate ion (ox) etc.
Tridentate ligands: Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.
Hexadentate ligands: Ligands which have six donor atoms per ligand, e.g. EDTA.
A place where you feel the CHEMISTRY

2. Chelating Ligands: Multidentate ligand simultaneously coordinating to a metal ion through more than one site is called
chelating ligand. Example: Ethylenediamine (NH2CH2CH2NH2)
These ligands produce a ring like structure called chelate. Chelation increases the stability of complex.
Stability of Chelating (ke-late Crab's cage) or Cyclic Complexes- Cyclic complexes i.e., chelates are more stable than open
complexes. This is because of reduced strain due to the formation of 5 or 6 membered ring including metal ion. Moreover in
cyclic complex, the ligand is attached with two or more bonds with metal ion, hence more bonds have to break. Due to this
reason cyclic complexes are more stable. The copper tetraammine complex is less stable than the copper ethylenediamine
complex although in both the cases nitrogen atom is the donor. Many cyclic complexes like Ni-dmg, Chlorophyll, Haemoglobin
are very stable towards dissociation, while the noncyclic complexes of these metal ions are less stable and dissociate easily.
Ambidentate ligand- Any ligand which has two or more donor atoms but only one donor atom is attached to the metal ion at a
time, during complex formation, is known as ambidentate ligand.
Example: NO2
-
and SCN
-
Here, NO2- can link through N as well as O while SCN- can link through S as well as N atom.
EAN Effective Atomic Number- total no. of electrons present in it.
EAN= atomic no. – oxidation state of metal + 2 x co-ordination number.
An ion with central metal atom having EAN equal to next inert gas will be more stable.
Organometallic compound- Any compound which contains at least one metal-carbon bond is called organometallic
compound. Eg. Pi bonded - Zeise’s salt K[PtCl3 η
2
-(CH2=CH2)],Ferrocene Sandwich compound (Cp) Fe (Cp)
[Fe η
5
-(C5H5)2] where Cp= cyclopentadiene
Sigma boded- (TEL) Tetra Ethyl Lead (C2H5)4 Pb, Grignard reagent - RMgX, Gilmann reagent - R2CuLi
Sigma and pi both bonded- Metal Carbonyls Ni(CO)4, Fe(CO)5 etc.
 Werner’s Theory
 Metals possess two types of valencies i.e. primary (ionizable) valency and secondary (nonionizable) valency.
 Secondary valency of a metal is equal to the number of ligands attached to it i.e. coordination number.
 Primary valencies are satisfied by negative ions, while secondary valencies may be satisfied by neutral,negative or
positive ions (ligands)
 Secondary valencies have a fixed orientation around the metal in space. i.e. it is directional. While primary valency is
non directional.
Example- [Co(NH3)6]Cl3 Primary Valencies = 3Cl
-
Secondary Valencies = 6NH3
Nomenclature of Complexes:
Keep in mind –
 Your left hand part is positive and right hand side is negative
 Positive ion is named first followed by negative ion.
 Negative ligands are named by adding suffix – o.
 Positive ligands are named by adding prefix – ium.
 Neutral ligands are named as such without adding any suffix or prefix.
 Ligands are named in alphabetical order (ABC).
 Name of the ligands is written first followed by name of metal with its oxidation number mentioned in roman numbers in
simple parenthesis.
 Number of the polysyllabic ligands i.e. ligands which have numbers in their name (like ethylenediamine), is indicated by
prefixes bis, tris, tetrakis etc,
 Number and name of solvent of crystallization if any, present in the complex is written in the end of the name of
complex.
 When both cation and anion are complex ions, the metal in negative complex is named by adding suffix -ate.

3.  In case of bridging ligands:
[Name of the groups to the left of bridging ligand (Oxidation state)] –μ – [Name of the groups to the right of bridging
ligand (Oxidation state)] – [Name of negative ion]
Some anions containing metal atoms
Metal Name of metal in anionic state
Copper Cuperate
Zinc Zincate
Aluminum Aluminate
Chromium Chromate Isomerism in coordination compounds
Tin Stannate
Cobalt Cobaltate 1. Structural Isomerism
Nickel Nickelate (i) Ionization
Gold Aurate (ii) Hydrate
Silver Argentate (iii) Linkage
Lead Plumbate (iv) Co-ordination sphere
Rhodium Rhodate 2. Stereo Isomerism
Iron Ferrate (i) Geometrical [cis- trans and fac- mer-]
Manganese Manganate (ii) Optical (mirror image)
1. Structural Isomerism
Ionization Isomerism: Exchange of ligands between coordinate sphere and ionization sphere
[Pt(NH3)4Cl2]Br2 & [Pt(NH3)4Br2]Cl2
Hydrate Isomerism: Exchange of water molecules between coordinate sphere and ionization sphere
[Cr(NH3)3(H2O)3]Br3 & [Cr(NH32)3(H2O)2 Br]Br2 H2O
Linkage Isomerism: Ambient legend binds from the different binding sites to the metal atom.
K2[Cu(CNS)4] & K2[Cu(SCN)4]
Coordination Isomerism: Exchange of the metal atom between coordinate sphere and ionization sphere
when both are complex ions.
[Cr(NH3)6][CoF6] & [Co(NH3)6][CrF6].
2. Stereoisomerism
a) Geometrical Isomerism: When two similar ligands are on adjacent position the isomer is called cis isomer
while they are on opposite positions, the isomer is called trans isomer.
1.Tetrahedral complex don’t show geometrical isomerism.
2. Ma3b3 type octahedral complexes show fac-, mer- isomerism.
3. [Mabcd]
n+/-
type square planar complex show 3 isomeric form.
b) Optical (mirror image) Isomerism: In order to show optical isomerism, the complex should form a
non-superimposable mirror image which rotates the place of polarized light in opposite direction.
To check whether a complex is optically active-look for Chiral Centre, absence of Plane of Symmetry,
NSIMI.
Keep in mind these are optically active-
Oh complexes containing polydentate ligands such as EDTA
4-
[M(aa)(ab)2]
n+/-
, [M(aa)2(ab)]
n+/-
, [M(aa)2(bb)]
n+/-
, [M(ab)3]
n+/-
, [M(aa)b2c2]
n+/-
, [M(aa)2bc]
n+/-
,
[M(aa)2b2]
n+/-
, [M(aa)3]
n+/-
, [Mabcdef]
n+/-
and [Ma2b2c2]
n+/-
Tetrahedral complexes having all four different groups bonded.
(aa) bidentate ligand with same donor atoms
(ab) bidentate ligand with different donor atoms
a,b,c,d,e different monodentate ligands.
Bonding in co-ordination compounds

4. Valence Bond Theory
1. This is based on magnetic moment calculations.
2. According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np or
ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral,
tetrahedral, and square planar.
Hybridization: Find out the hybridization of central metal ion using following steps:
Write down the electronic configuration of metal atom.
Find out oxidation state of metal atom.
Write down the electronic configuration of metal ion.
b) These hybridized orbitals are allowed to overlap with ligand orbitals that can donate
electron pairs for bonding.
Strong field ligands cause the pairing of electrons.
Strong Field Ligands: CO, CN-, NO2-, en, py, NH3.
Weak Filed Ligands: H2O, OH-, F-, Cl-, Br-,I -
When the d orbital taking part in hybridization is inside the s and p orbital taking part in hybridization with
respect to the nucleus, it is called an inner orbital complex. Example: d2sp3 hybridization of
[Co(NH3)6]3+ involves 3d, 4s and 4p orbital, hence it is an inner orbital complex.
When the d orbital taking part in hybridization outside the s and p orbital taking part in hybridization with
respect to the nucleus, it is called an outer orbital complex.
Example: sp3d2 hybridization of [CoF6]3- involves 4d, 4s and 4p orbital, hence it is an inner orbital complex.
Coordination Number Hybridization Geometry
4
4
6
sp3 or sd3
dsp2
d2sp3 (inner orbital) & sp3d2
(outerorbital)
Tetrahedral
Square Planar
Octahedral
Apps- Hybridization, inner orbital-outer orbital complex, geometry of the complex, Magnetic Properties
Magnetic moment µ =√n(n+2) where n is no. of unpaired electrons
Diamagnetic: All the electrons paired. (µ = 0)
Paramagnetic: Contains unpaired electrons. (µ ≠ 0)
Limits- it fails to predict
Strong field and weak field ligand, why pairing occurs in presence of strong ligands, color, reaction
mechanism, distortion, structure of [Cu(NH3)4]2+ ion etc.
Crystal Field Theory-
1. It assumes the ligands to be point charges and there is electrostatic force of
attraction between ligands and metal atom or ion.
2. It is theoretical assumption.
3. When ligands approaches the metal in an octahedron environment, the degeneracy of metal d-
orbitals breaks and two new sets of d orbitals form, a set of three d-orbitals(dxy, dyz, dzx) is t2g and a
set of two d-orbitals (dx
2
-y
2
, and dz
2
) is eg .
4. The energy difference b/w t2g and eg set is called Δo crystal field splitting energy.
5. In Octahedral Complexes: eg orbital are of higher energy than t2g orbital.
6. Tetrahedral Complexes: e orbitals are of lower energy than t2 orbitals.
7. Strong field ligand causes greater repulsion and thus results in the formation of low spin (spin paired)
complexes by pairing of electrons.
8. Weak field ligands result in the formation of high spin complexes (spin free).
Spectrochemical series gives the Order of strength of ligands:
CO > CN- > NO2- > en > py = NH3 > H2O > OH- > F- > Cl- > Br- >I-

5. Splitting of d orbitals in tetrahedral crystal field Splitting of d orbitals in octahedral field
Δt = (4/9) Δo Δsp = 1.74 Δo
Crystal Field Stabilization Energy for octahedral field
[ - 0.4 ( electrons in t2g set) + 0.6 (electrons in eg set) ]
Pairing energy (P) - The energy require to force two unpaired electron in one orbital.
If Δo > P, low spin complex, Δo < P, high spin complex, Δo = P both L.S. and H.S. complex equally exists
Applications of CFT- Color, magnetic property, spin paired (L.S.) or spin free (H.S.),stability of complex,
more the CFSE more will be the stability
Magnetic Properties: Complexes with unpaired electrons are paramagnetic while with no unpaired electron
are diamagnetic.
Spin paired: All electrons paired. Spin free: Contains unpaired electrons.
Compound must contain free electrons in order to show color.
Limits- only d- orbital of metal is focused not s and p.
Covalent character b/w metal ligand bond is ignored, not consider the π bonding, complexes like Cr(CO)6,
Ni(CO)4 in which metal is in zero oxidation state and ligand is neutral where is the electrostatic attraction?
• Metal carbonyls:
a) Metal carbonyls are homoleptic complexes in which carbon monoxide (CO) acts as the ligand.
b) Example: Ni(CO)4
c) The metal-carbon bond in metal carbonyls possess both sigma and pi character.
d) The M–C σ bond is formed by the donation of lone pair of electrons from the carbonyl carbon into a vacant
orbital of the metal.
e) The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the
vacant antibonding π * orbital of carbon monoxide.
f) The metal to ligand bonding creates a synergic effect which strengthens the bond
between CO and the metal. This type of bonding is called synergic bonding or back pi bonding.
Factors affecting stability Complexes-
(i) Basic nature of the ligand- Greater the basic nature of the ligand greater will be the stability. The
copper complex with CN– is more stable than the complex with NH3 (less basic).
(ii) Charge on central metal ion- Greater the charge on central metal ion greater will be the stability. [Fe
(CN)6]
3–
is more stable than [Fe (CN)6]
4–
due to the higher charge on Fe.
(iii) Number of ring structures in complex- If a ring is formed during complexion, it provides extra
stability. That is why chelates are more stable.
βn = K1.K2.K3…………Kn βn is overall formation constant or overall stability constant and Kn is stepwise
formation constant or stepwise stability constant.
Dissociation constant or instability constant Kd = 1/βn

6. Apps of coordination Chemistry
In Industry
(i) Photography : In photography, the excess of silver salts has to be remove to fix the image on the negative
and to prevent their further reduction. For this purpose the negative is immersed in a bath containing sodium
thiosulphate solution, where excess of silver forms the soluble complex and is washed away.
(ii) In electroplating : Cyanocomplexes of copper, silver, gold etc, are used for electroplating of these metals.
Since the metal complexes release the metal ions slowly, therefore, a thin and uniform coating of the metal
can be deposited on desired object with the help of electroplating.
In medicine-
Many coordination compounds are successfully used as chemotheropeutic drugs.
(i) cis-platin [PtCl2(NH3)2] is used in the treatment of cancer.
(ii) Metal complexes of Ni, Fe, Co with some nitrogen and sulphur containing ligands are very good
antituberculosis agents.
(iii) EDTA as complexing agent for (Pb) lead poisoning.
In Metallurgy- Ag2S + NaCN → Na[Ag(CN)2] + Na2S
Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + Ag
Ligands Name Charge
CH3COO
-
Acetato -1
CN
-
Cyano -1
Br
-
Bromo -1
Cl-
Chloro -1
F-
OH-
H
-
N
3-
NH2-
NH2
-
N3
-
C2O4
2-
CO3
2-
SO4
2-
SO3
2-
S2O3
2-
HS
-
S
2-
O2
-
O2
2-
O
2-
NO2
-
ONO-
CN-
CNO
-
SCN
-
NCS
-
EDTA4-
NH2CH2COO-
C5H5N
P(Ph)3
C2H4
NH2-NH2
NH2OH
N2 / O2
CO
NO
H2O
NH3
CS
NS
CH3NH2
NO+
NH2NH3+
Fluoro
Hydroxo
Hydrido
Nitrido
Imido
Amido
azido
Oxalato (ox)
Carbonato
Sulphato or Sulfato
Sulphito or Sulfito
Thiosulfato
Mercapto
Sulphido or Sulfido
Superoxo
Peroxo
Oxo
Nitro or nitrito –N
Nitrito or nitrito –O
Cyano
Cyanato
Thiocyanato or thiocyanato –S
Isothiocyanato or thiocyanato –N
Ethylenediaminetetraacetato ion
Glycinato (gly)
(dmg)
-1
dimethylglyoximato
(acac)
-1
acetylacetonato
Pyridine (py)
Triphenyl phosphine
Ethylene
Hydrazine
Hydroxylamine
Dinitrogen / Dioxygen
Carbonyl
Nitrosyl
Aqua
Ammine
Thiocarbonyl
Thionitrosyl
Methyl amine
(en) ethylene diamine
(pn)
(tn)
(bn)
(i-bn)
(bpy) or (bipy)
(o-phen) or phen
(dien)
(trien)
(tren)
(tetraen)
Nitrosonium
Hydrazinium
-1
-1
-1
-3
-2
-1
-1
-2 (bidentate)
-2
-2
-2
-2
-1
-2
-1
-2
-2
-1
-1
-1
-1
-1
-1
-4 (hexadentate)
-1 (bidentate)
-1 (bidentate)
-1 (bidentate)
Neutral (monodentate)
Neutral (monodentate)
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral
Neutral (bidentate)
Neutral (bidentate)
Neutral (bidentate)
Neutral (bidentate)
Neutral (bidentate)
Neutral (bidentate)
Neutral (bidentate)
Neutral (tridentate)
Neutral (tetradentate)
Neutral (tetradentate)
Neutral (pentadentate)
Positive
Positive