CBSE Class 12 Chemistry Coordination Compounds Notes Set A

Coordination compounds – introduction :

The compounds which contain dative bonds between metal atom and surrounding species is called co-ordination compounds.

Examples- K₄[Fe(CN)₆]; [ Cu(NH₃)₄]SO₄; Ni(CO)₄ ; K₄[Fe(CN)₆], [CuNH₃)₄]SO₄
Coordination complex: A structure containing a metal (usually a metal ion) bonded (coordinated) to a group of surrounding molecules or ions. They retain their identities even when dissolved in water.

Double Salts-
The addition compounds which are stable in solid state but are broken down into individual constituents ions when dissolved in water eg: mohr’s salt,potash alum etc.

Coordination or complex compounds –
The addition compounds which retain their identity in solid as well as solution state . Complex ions such as [Fe(CN)₆]^4– of K₄[Fe(CN)₆], do not dissociate into Fe²⁺ and CN– ions. Some more eg: chlorophyll, hemoglobin etc.

a) Central metal atom or ion –
the metal ion surrounded by number of ions or molecules.
eg:- K₄[Fe(CN)₆] , Fe²⁺ is central metal ion.

b) Ligands –
a molecule,ion or group that coordinates to the central metal atom or ion bound in a definite geometrical arrangement in a complex . e.g.-H₂O, NH₃, CO,
A ligand uses a lone pair of electrons (Lewis base) to bond to the metal ion (Lewis acid)

c) Coordination number-
The number of coordinating units/ligands around the metal ion.
The no. of ligand donor atoms to which the metal is directly bonded through sigma bonds only. It is commonly 4 or 6.

d) Coordination sphere or Coordination entity -
the square brackets enclosing the central metal ion and ligands

e) Counter ions-the ions present outside the coordination sphere
e.g.- K+ in K₄[Fe(CN)₆] OR 3Cl- in [Co(NH₃)₆]Cl₃

Co-ordinate covalent bond: metal-ligand bond 

Ligands may be classified as a………..

a)Monodentate/Unidentate: Ligands bound to the central metal atom/ion through a single donor atom. e.g.- Cl- ; H₂O ; NH₃ ; NO₂-.

b) Didentate: Ligates through two donor atoms.
e.g.- C₂O4 ^2- (ox); H₂NCH₂CH₂NH₂(en)

c) Polydentate: which ligates through two or more donor atoms present in a single ligand.
e.g.- (EDTA)^4-

d) Chelating ligands: Di- or polydentateligands that usestwo or more donor atoms to bind to a single metal ion to form ring- like complexes.
e.g.- (Ox); (edta)

e) Ambidentate ligand: A ligand that can ligate through two different atoms, one at a time.
e.g.- NO₂- ; SCN^-

MAIN POSTULATES WERNER’S THEORY OF COORDINATION COMPOUNDS

i) In coordination compounds metals show two types of linkages or valencies- Primaryand Secondary.

ii) The primary valencies are ionisable and are satisfied by negative ions. 

iii) The secondary valencies are non- ionisable and are satisfied by neutral molecules or negative ions. The secondary valence is equal to the C.N and is fixed for a metal.

iv) The ions or groups bound by secondary linkages to the metal have characteristic spatialarrangements corresponding to different coordination nos.

NAMING OF MONONUCLEAR CO-ORDINATION COMPOUNDS
The principle of additive nomenclature is followed while naming the coordination compounds. The following rules are used
i. The cation is named first in both positively and negatively charged coordination entities.

ii. The ligands are named in an alphabetical order before the name of the central atom/ion

iii. The name of the anionic ligands end in –o, those of neutral and cationic ligands are he same except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO.these are placed within enclosing marks .

iv. When the prefixes mono, di, tri, etc., are used to indicate the number of theindividual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris ,tetrakis are used, the ligand to which theyrefer being placed in parenthesis.

v. Oxidation state of the metal in cation, anion, or neutral coordination entity isindicated by roman numeral in parenthesis.

vi. If the complex ion is a cation , the metal is same as the element.

vii. The neutral complex molecule is named similar to that of the complex cation.

SOME COMMON EXAMPLES :-

[Cr(NH₃)₃(H₂O)₃]Cl₃ is named as: triamminetriaquachromium(III) chloride
[Co(H₂NCH₂CH₂NH₂)₃]₂(SO₄)₃ : tris(ethane-1,2-diammine)cobalt(III) sulphate

ISOMERISM ;- Isomers have the same molecular formula, but their atoms are arranged either in a different order (structural isomers) or spatial arrangement (stereoisomers). 

(a) Stereoisomerism
(i) Geometrical isomerism (ii) Optical isomerism

(b) Structural isomerism
(i) Linkage isomerism (ii) Coordination isomerism
(iii) Ionisation isomerism (iv) Solvate isomerism

(i) Geometrical isomerism:-
It arises in heteroleptic complexes due to different possible geometrical arrangements of ligands.

(ii) Optical isomerism :

Optical isomers are those isomers which are non superimposable mirror images.

OPTICAL ISOMERISM:

Optical isomers (enantiomers) are mirror images which cannot be superimposed on one another.The molecules or ions which cannot be superimposed are called chiral. 

STRUCTURAL ISOMERISM – 

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