Online Notes on "Coordination Compounds" (Chapter=>9 ) Part 1

These are Online Notes on "Coordination Compounds" (Chapter=>9 ) Part 1 for practice of CBSE BOARD, CBSE NEET, CSIR NET Chemical Sciences etc.

1. Difference between coordination compound and double bond:

Coordination compound	Double salt
A coordination compound contains a	When two salts in stoichiometric ratio
central metal atom or ion surrounded	are  crystallised  together  from  their
by number of oppositely charged ions	saturated  solution  they  are  called
or  neutral molecules. These ions or	double salts
molecules  re  bonded  to  the  metal
atom or ion by a coordinate bond.
Example: K4[Fe(CN)6]	Example:FeSO4.(NH4)2SO4.6H2O
	(Mohr’s salt)
They  do  not  dissociate  into  simple	They dissociate into simple ions when
ions when dissolved in water.	dissolved in water.

2.           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.

3.           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. E.g.: in K4[Fe(CN)6], Fe²+  is the central metal ion.

4.           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. 

Example: H2O, CN-, NO+ etc.

5.           Donor atom:    An atom of the ligand attached directly to the metal

is called the donor atom. For example: in the complex K4[Fe(CN)6], 

carbon is a donor atom.

6.           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. For example: in the complex K4[Fe(CN)6], the coordination number of Fe is 6.

7.           Coordination sphere: The central atom/ion and the ligands  attached

to it are enclosed in square bracket and is collectively termed as the coordination sphere. For example: in the complex K4[Fe(CN)6], [Fe(CN)6]4-  is the coordination sphere.

8.           Counter ions:   The ions present outside the coordination sphere   are

called counter ions. For example: in the complex K4[Fe(CN)6], K+ is the counter ion.

9.           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. For example: [PtCl4]2- is square planar, Ni(CO)4 is tetrahedral while [Cu(NH3)6]3+ is octahedral.

10.           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.

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

12.           Unidentate ligands: The ligands whose only one donor atom is bonded to metal atom are called unidentate ligands. Example: H2O, NH3, CO, CN-

13.           Didentate ligands: The ligands which contain two donor atoms or ions through which they are bonded to the metal ion. For example: ethylene diamine (H2NCH2CH2NH2) has two nitrogen atoms, oxalate ion

æC OO- ö

ç ÷           

è  !         ø

ç COO- ÷

(OOC- COO)- has two oxygen atoms which can bind with the metal atom.

14.           Polydentate ligand: When several donor atoms are present in a single ligand, the ligand is called polydentate ligand. For example:    in

N(CH2CH2NH2)3, the ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

15.           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+.

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