Chemistry Introduction and Werner's Theory

Topics Covered :

● Introduction
● Werner's Theory
● Difference between Double Salt and Complex

Introduction

`=>` We know 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.

`=>` 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 vital components of biological systems.

● Chlorophyll, haemoglobin and vitamin `color{red}(B_(12))` are coordination compounds of magnesium, iron and cobalt respectively.

● 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.


1. `color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
•Coordination compounds are the compounds in which the central metal atom is linked to a number of ions or neutral molecules by coordination bonds, i.e., by donation of lone pairs of electrons by these ions or neutral molecules to the central metal atom.

•The branch of inorganic chemistry dealing with the study of coordination compounds is known as coordination chemistry.

Werner’s Theory of Coordination Compounds :

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

`=>` He prepared and characterised a large number of coordination compounds and studied their physical and chemical behaviour by simple experimental techniques.

`=>` Werner proposed the concept of a `text(primary valence)` and a `text(secondary valence)` for a metal ion.

● Binary compounds such as `color{red}(CrCl_3, CoCl_2)` or `color{red}(PdCl_2)` have 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.

`color{red}(1 mol CoCl_3.6NH_3 ("Yellow") "gave" 3 mol AgCl)`
`color{red}(1 mol CoCl_3.5NH_3 "(Purple) gave" 2 mol AgCl)`
`color{red}(1 mol CoCl_3.4NH_3 "(Green) gave" 1 mol AgCl)`
`color{red}(1 mol CoCl_3.4NH_3 "(Violet) gave" 1 mol AgCl)`

`=>` These observations, together with the results of conductivity measurements in 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 which does not dissociate under the reaction conditions.

`color{green}("Secondary Valence ")` : 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.

`color{red}("Note ")` : The last two compounds in Table 9.1 have identical empirical formula, `color{red}(CoCl_3 .4NH_3)`, but distinct properties. Such compounds are termed as `color{green}("isomers")`.

`=>` Werner in `1898`, propounded his theory of coordination compounds.

`color{green}("Postulates ")` :

(i) In coordination compounds metals show two types of linkages (valences)-primary and secondary.

(ii) The primary valences are normally ionisable and are satisfied by negative ions.

(iii) The secondary valences are non ionisable.

● These are satisfied by neutral molecules or negative ions.

● The secondary valence is equal to the coordination number and is fixed for a metal.

(iv) The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different
coordination numbers.

● In modern formulations, such spatial arrangements are called `color{green}("coordination polyhedra")`.

● The species within the square bracket are `color{green}("coordination entities")` or `color{green}("complexes")` and the ions outside the square bracket are called `color{green}("counter ions")`.

`=>` He also postulated that octahedral, tetrahedral and square planar geometrical shapes are more common in coordination compounds of transition metals.

● `color{red}("Example ")` : `color{red}( [Co(NH_3)_6]^(3+), [CoCl(NH_3)_5]^(2+))` and `color{red}([CoCl_2(NH_3)_4]^+)` are octahedral entities.

`color{red}([Ni(CO)_4])` and `color{red}([PtCl_4]^(2–))` are tetrahedral and square planar, respectively.
Q 3031601522

On the basis of the following observations made with aqueous solutions, assign secondary valences to metals in the following compounds:

Solution:

(i) Secondary 4 (ii) Secondary 6
(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4

Difference between a Double Salt and a Complex :

`color{green}("Double Salt" )`: Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio.

● They differ in the fact that double salts such as carnallite, `color{red}(KCl .MgCl_2 .6H_2O)`, Mohr’s salt, `color{red}(FeSO_4 .(NH_4)_2SO_4 .6H_2O)`, potash alum, `color{red}(KAl(SO_4)_2 .12H_2O)`, etc. dissociate into simple ions completely when dissolved in water.

`color{green}("Complex ")` Complex ions such as `color{red}([Fe(CN)_6]^(4-))` of` color{red}(K_4Fe(CN)_6)`, do not dissociate into `color{red}(Fe^(2+))` and `color{red}(CN^-)` ions.

 
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