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Revision Notes on Coordination Compounds

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. 

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.

  • 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

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]Br &  [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]ClWhere,
    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.

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-
    ?

  • Octahedral Complexes: eg orbital are of higher energy than t2g orbital.
     

Splitting of d orbitals in octahedral crystal field
 

  • Tetrahedral Complexes: eg orbitals are of lower energy than t2g orbitals.

 

Splitting of d orbitals in tetrahedral crystal field 

 Δt = (4/9) Δo

Crystal Field Stabilization Energy:

System

High Spin

Low Spin

 

Electronic Configuration

CFSE

Electronic Configuration

CFSE

Octahedral Complex

d4

t2g3 eg1

-(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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