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, C₂O₄^2-(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]

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]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.

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, NH_3.
  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:

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^-
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• Octahedral Complexes: e_g orbital are of higher energy than t_2g orbital.
   

• Tetrahedral Complexes: e_g orbitals are of lower energy than t_2g orbitals.

 Δ_t = (4/9) Δ_o

Crystal Field Stabilization Energy:

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