Coordination Compounds Chapter 9 Class 12 Chemistry NCERT Textbook PDF

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Coordination Compounds

Chapter 9: Coordination Compounds

In the previous Unit, we learned that the transition metals form a large number of complex compounds in which the metal atoms are bound to a number of anions or neutral molecules by sharing electrons.

In modern terminology, such compounds are called coordination compounds.

The chemistry of coordination compounds is an important and challenging area of modern
inorganic chemistry.

New concepts of chemical bonding and molecular structure have provided insights into the functioning of these compounds as vital components of biological systems.

Chlorophyll, hemoglobin, and vitamin B12 are coordination compounds of magnesium, iron, and cobalt respectively.

A variety of metallurgical processes, industrial catalysts, and analytical reagents involve the use of coordination compounds.

Coordination compounds also find many applications in electroplating, textile dyeing, and medicinal chemistry.

Alfred Werner (1866-1919), a Swiss chemist was the first to formulate his ideas about the structures of coordination compounds.

He prepared and characterized a large number of coordination compounds and studied their physical and chemical behavior by simple experimental techniques.

Werner proposed the concept of a primary valence and a secondary valence for a metal ion. Binary compounds such as CrCl3, CoCl2, or PdCl2 have a primary valence of 3, 2, and 2 respectively.

In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution.

1 mol CoCl3 .6NH3 (Yellow) gave 3 mol AgCl

1 mol CoCl3 .5NH3 (Purple) gave 2 mol AgCl

1 mol CoCl3 .4NH3 (Green) gave 1 mol AgCl

1 mol CoCl3 .4NH3 (Violet) gave 1 mol AgCl

These observations, together with the results of conductivity measurements in the solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to
the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 9.1.

where the atoms within the square brackets form a single entity that does not dissociate under the reaction conditions. Werner proposed the term secondary valence for the number of groups bound directly to the metal ion; in each of these examples the secondary valences are six

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NCERT Solutions Class 12 Chemistry Chapter 9 Coordination Compounds

Q1. Explain the bonding in coordination compounds in terms of Werner’s postulates.
Ans :

( a ) A metal shows two kinds of valencies viz primary valency and secondary valency. Negative ions satisfy primary valencies and secondary valencies are filled by both neutral ions and negative ions.
( b ) A metal ion has a fixed amount of secondary valencies about the central atom. These valencies also orient themselves in a particular direction in the space provided to the definite geometry of the coordination compound.
( c ) Secondary valencies cannot be ionized, while primary valencies can usually be ionized.

Q2. FeSO4 solution mixed with (NH4)2SO4 solution in 1:1 molar ratio gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous ammonia in 1:4 molar ratio does not give the test of Cu2+ ion. Explain why.

Ans :

FeSO4 solution when mixed with (NH4)2SO4in a 1: 1 molar ratio produces a double salt FeSO4(NH4)2SO4-6H2O. This salt is responsible for giving the Fe2+.
CuSO4 mixed with aqueous ammonia in the ratio of  1:4 gives a complex salt.  The complex salt does not ionize to give Cu2+, hence failing the test.

Q3. Explain with two examples each of the following: coordination entity, ligand, coordination number, coordination polyhedron, homoleptic and heteroleptic.

Ans :

( a ) Ligands – they are neutral molecules or negative ions bound to a metal atom in the coordination entity. Example- Cl,  OH
( b ) Coordination entity –  they are electrically charged radicals or species. They constitute a central ion or atom surrounded by neutral molecules or ions. Example –  [ Ni(CO)4], [COCL3(NH3)3]
( c ) Coordination number– it is the number of bonds formed between ligands and central atom/ion.

Example :    ( i ) In K2[PtCl6], 6 chloride ions are attached to Pt in the coordinate sphere. Thus, 6 is the coordination number of Pt.
( ii ) In [Ni(NH3)4]Cl2, the coordination number of the central metal ion (Ni) is 4.
( d ) Coordination polyhedron –  it is the spatial positioning of ligands that are directly connected to the central atom in the coordination sphere.

( v ) Heteroleptic: they are complexes with their metal ion being bounded to more than one kind of donor group. Example – [ Co(NH3)4Cl2]+ , [ Ni(CO)]
( vi ) Homoleptic: they are complexes with their metal ion being bounded to only one type of donor. Example – [ PtCl4]2- , [ Co(NH3)6]3+

Q4. What is meant by unidentate, bidentate, and ambidentate ligands? Give two examples for each.

Ans :

( i ) Unidentate ligands: these are ligands with one donor site. Example Cl, NH3
( ii )  Ambidentate ligands: these are ligands that fasten themselves to the central metal ion/ atom via two different atoms.
Example NO2or ONO, CN or NC
( iii ) Bidentate – these are ligands with two donor sites.
Example – Ethane-1,2-diamine, Oxalate ion ( C2O42- )

Q5. Specify the oxidation numbers of the metals in the following coordination entities:
(i) [Co(H2O)(CN)(en)2]2+

(ii) [CoBr2(en)2]+

(iii) [PtCl4]2–

(iv) K3[Fe(CN)6]

(v) [Cr(NH3)3Cl3]  

Ans :

( i ) [Co (H2O) (CN) (en)2+
=>  x + 0  + ( -1)  + 2 (0) = +2
x -1 = +2
x = +3

( ii )  [ Co Br(en)1+
=> x + 2 ( -1 ) + 2 ( 0 ) = +1
x  – 2 = +1
x = +3

( iii) [ PtCl2-
=> x + 4 ( -1 ) = -2
x = +2

( iv ) K[Fe (CN)]
=>  [ Fe ( CN )6]3-
=> x + 6 ( -1 ) = -3
x = +3

( v ) [ Cr(NH3)3Cl]
=> x + 3( 0 ) + 3 ( -1 ) = 0
x – 3 = 0
x = 3

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