CO-ORDINATION CHAPTER NOTES-XII

                

                                   Coordination Compounds

INTRODUCTION;-

 A coordination compound contains a central metal atom or ion surrounded by number of oppositely charged ions or neutral molecules. These ions or molecules are bonded to the metal atom or ion by a coordinate bond.                Example: K₄[Fe(CN)₆]

 They do not dissociate into simple ions when dissolved in water.

Double salt

·         When two salts in stoichiometric ratio are crystallised together from their saturated solution they are called double salts  Example:FeSO₄.(NH₄)₂SO₄.6H₂O (Mohr’s salt)

·         They dissociate into simple ions when dissolved in water.

Coordination entity:

A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.  Example: In K₄[Fe(CN)₆], [Fe(CN)₆]^4- represents coordination entity.

Central atom or ion:

In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.

Example: In K₄[Fe(CN)₆], Fe^²+ is the central metal ion.

Ligands:

A molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is called ligand. It may be neutral, positively or negatively charged.Examples: H₂O, CN^-, NO⁺ etc.These are gereally donate celectron to attached  metal  and hence is called the donor atom.

 Example: In the complex K4[Fe(CN)₆], CN is a donor atom.

Coordination number:

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.

Example: In the complex K₄[Fe(CN)₆], the coordination number of Fe is 6.

Coordination sphere:

The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the coordination sphere. Example: In the complex K₄[Fe(CN)_6], [Fe(CN)₆]^4- is the coordination sphere.

Counter ions:

The ions present outside the coordination sphere are called counter ions. Example: In the complex K₄[Fe(CN)₆], K⁺ is the counter ion.

Coordination polyhedron:

The spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral.

 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.

Denticity:

The number of ligating (linking) atoms present in ligand is called denticity.

Unidentate ligands:The ligands whose only one donor atom is bonded to metal atom are called

unidentate ligands. Examples: H₂O, NH_3, CO, CN

Didentate ligands:The ligands which contain two donor atoms or ions through which they are bonded

to the metal ion.Examples: Ethylene diamine (H₂NCH₂CH₂NH₂) has two nitrogen atoms, C₂O₄^2-(oxalate ion) has two oxygen atoms which can bind with the metal atom.

Polydentate ligand:When several donor atoms are present in a single ligand, the ligand is called

polydentate ligand.Examples: In N(CH₂CH₂NH₂)₃, the ligand is said to be polydentate and

Ethylenediaminetetraacetate ion (EDTA) is an important hexadentate ligand. It can

bind through two nitrogen and four oxygen atoms to a central metal ion.

Chelate:

An inorganic metal complex in which there is a close ring of atoms caused by attachment of a ligand to a metal atom at two points. An example is the complex ion formed between ethylene diamine and cupric ion,[Cu(NH₂CH₂NH₂)₂]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. 

Chelation and chelating ring

The ligand may contain two donor atoms positioned in such a way that a five or six membered ring is formed with the metal ion, then it is called didentate chelating ligand and the ring is called chelating ring, the resulting complex is called a metal chelate and this property is called chelation.

Important characteristics of chelate

·         They form more stable complexes than the unidentate analogs.

·         Greater the denticity of the ligand, more stable is the complex formed.

·         Ligands with large groups form unstable rings than the ligands with smaller groups due to steric hinderance.

Importance of chelates

·         It is used in softening of hard water.

·         It is used in the separation of lanthanoids and actinoids.

·         It is used in the detection of metal ions in qualitative analysis.

Ambidentate ligand:

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