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Ligands: an ion or molecule capable of donating a pair of electrons to the central atom via a donor atom.

  • Unidentate ligands: Ligands with only one donor atom, e.g. NH3, Cl-, F- etc.
  • Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine, C2O42-(oxalate ion) 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. 

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.
     Notes | EduRev

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.
     Notes | EduRev
  • Secondary valencies have a fixed orientation around the metal in space.

[Co(NH3)6]Cl3
Primary Valencies = 3 Cl-
Secondary Valencies = 6 NH3
Coordination Sphere =  [Co(NH3)6]3-

Nomenclature of Complexes:

  • 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.
  • 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, is indicated by prefixes bis, tris 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.
  • 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]
Ligands
Name
Negative
CH3COO-
Acetato
CN-
Cyano
Br-
Bromo
Cl-
Chloro
F-
Fluoro
OH-
Hydrido
N3-
Nitrido
C2O42-
Oxalato
SO32-
Sulfito
O2-
Superoxo
O22-
Peroxo
O2-
Oxo
NH2-
Imido
SO42-
Sulphato
S2O32-
Thiosulfato
HS-
Mercapto
Positive
NO+
Nitrosonium
NH2NH3+
Hydrazinium
Neutral
H2O
Aqua
NH3
Ammine
CO
Carbonyl
CH3NH2
Methylamine
NO
Nitrosyl
C5H5N
Pyridine

Isomerism in coordination compounds
 Notes | EduRev

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].
  • Ligand Isomerism: Different isomers of the same ligands attached to the metal.
    [Co(pn)2Br]Cl2 & [Co(tn)2Br]Cl2 Where,
    pn = 1,2- Diaminopropane
    tn = 1,3-Diaminopropane.

Stereoisomerism:
a.Geometrical Isomerism: When two similar ligands are on adjacent position the isomer is called cis isomer while hen they are on opposite positions, the isomer is called trans isomer.
 Notes | EduRev
b.Optical Isomerism: In order to show optical isomerism, the complex should form a non superimposible mirror image which rotates the place of polarized light in opposite direction.

Valence Bond Theory:
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.
  • Write down the configuration of complex to find out hybridization.
  • 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.

Geometry:

Coordination Number
Hybridization
Geometry
4
sp3
Tetrahedral
dsp2
Square Planar
6
d2sp3 & sp3d2
Oct

Magnetic Properties:

  • Diamagnetic: All the electrons paired.
  • Paramagnetic: Contains unpaired electrons.

Spin:

  • Spin paired: All electrons paired.
  • Spin free: Contains unpaired electrons.

Colour:
Compound must contain free electrons in order to show colour.

Crystal Field Theory:
Strong field ligand causes greater repulsion and thus results in the formation of low spin complexes by pairing of electrons.

  • Weak field ligands result in the formation of high spin complexes
  • Order of strength of ligands: CO > CN- > NO2- > en > py = NH3 > H2O > OH- > F> Cl- > Br- >I-
     Notes | EduRev
  • Octahedral Complexes: eg orbital are of higher energy than t2g orbital.
     Notes | EduRev
  • Tetrahedral Complexes: eg orbitals are of lower energy than t2g orbitals.
     Notes | EduRev 

 Δt = (4/9) Δo

Crystal Field Stabilization Energy:

System
High Spin
Low Spin
 
Electronic Configuration
CFSE
Electronic Configuration
CFSE
Octahedral Complex
d4
t2geg1
-(3/5)Δ0
t2g4 eg0
-(8/5)Δ0+P
d5
t2g3 eg2
0
t2g5 eg0
-(10/5)Δ0+2P
d6
t2g4 eg2
-(2/5)Δ0+P
t2g6 eg0
-(12/5)Δ0+3P
d7
t2g5 eg2
-(4/5)Δ0+2P
t2g6 eg1
-(9/5)Δ0+3P
Tetrahedral Complexes
d4
eg2  t2g2
-(2/5)Δt
eg4 t2g0
-(12/5)Δt +2P
d5
eg2  t2g3
0
eg4 t2g1
-2 Δt +2P
d6
eg3  t2g3
-(3/5)Δt +P
eg4 t2g2
-(8/5)Δt+2P 

Magnetic Properties: Complexes with unpaired electrons are paramagnetic while with no unpaired electron are diamagnetic.

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