Basic Terms and Types of Ligands Class 12 Notes | EduRev

Chemistry Class 12

Created by: Mohit Rajpoot

Class 12 : Basic Terms and Types of Ligands Class 12 Notes | EduRev

The document Basic Terms and Types of Ligands Class 12 Notes | EduRev is a part of the Class 12 Course Chemistry Class 12.
All you need of Class 12 at this link: Class 12

KEY CONCEPTS 

Molecular / Addition Compound

Molecular / Addition compounds are formed when stoichiometric amounts of two or more simple compounds join together. Molecular / Addition compounds are of two types.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Molecular compound

Double salts

Those which retain their identity in solutions are called double salts. For example.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Complex compounds

Those which loose their identity in solution (complexes). For example.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

When crystals of carnallite are dissolved in water, the solution shows properties of K+ , Mg2+  and Cl- ions. In a similar way, a solution of potassium alum shows the properties of K+ , Al3+  and SO42- ions. These are both examples of double salts which exist only in the crystalline state. When the other two examples of coordination compounds are dissolved they do not form simple ions, Cu2+ ,Fe2+  and CN-, but instead their complex ions are formed.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Complex Compounds

Representation of Complex Ion 

Basic Terms and Types of Ligands Class 12 Notes | EduRev 

when

M = Central Metal atom /ion (usually of d-block)

L = Ligand

x = No. of ligands

Basic Terms and Types of Ligands Class 12 Notes | EduRev = charge on coordination

Outside region apart from coordination sphere is called ionisation sphere.

1. Central metal atom/ion: Central ion acts as an acceptor (Lewis acid) and has to accommodate electron pairs donated by the donor atom of the ligand, it must have empty orbitals. This explains why the transition metals having empty d-orbitals form co-ordination compounds readily. Thus, in complexes [Ni(NH3)6] and [Fe(CN)6]3-, Ni and Fe respectively are the central metal ions.


Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Central Metal Atom

2. Ligands: Species which are directly linked with the central metal atom/ ion in a complex ion are called ligands. The ligands are attached to the central metal atom /ion through co-ordinate or dative bond free ligands have at least one lone pair.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

The lignads are thus Lewis bases and the central metal ions and n atoms are Lewis acids.


Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Ligands


Ligands can be of following types depending on the number of donor atoms pesent in them. 
(i) Mono / Unidentate Ligands They have one donor atom, i.e., they can donate only one electron pair to the central metal atom /ion eg., F-, Cl-, Br-, H2O, NH3, CN-,NO2-, OH-

CO etc.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Unidentate Ligands


(ii) Bidentate Ligands Ligands which have two donor atoms and have the ability to link with the central metal atom /ion at two position are called bidentate ligands e.g.

Basic Terms and Types of Ligands Class 12 Notes | EduRevBasic Terms and Types of Ligands Class 12 Notes | EduRevBasic Terms and Types of Ligands Class 12 Notes | EduRev

Basic Terms and Types of Ligands Class 12 Notes | EduRevBasic Terms and Types of Ligands Class 12 Notes | EduRevBasic Terms and Types of Ligands Class 12 Notes | EduRev

(iii) Tridentate Ligands Ligands having three donor atoms are called tridentate ligands. Examples are

Basic Terms and Types of Ligands Class 12 Notes | EduRev

diethylene triamine (dien)


Basic Terms and Types of Ligands Class 12 Notes | EduRev

(iv) Tetradentate Ligands These ligands possess four donor atoms. Examples are

Basic Terms and Types of Ligands Class 12 Notes | EduRevBasic Terms and Types of Ligands Class 12 Notes | EduRev

(v) Pentadentate Ligands They have five donor atoms. For example, ethylenediamine triacetate ion.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

(vi) Hexadentate Ligands They have six donor atoms. The most important example is ethylenediamine tetraacetate ion.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

(Vii) Ambidentate ligands: There are certain ligands which have two or more donor atoms but in forming complexes, only one donor atom is attached to the metal / ion. Such ligands are called ambidentate ligands. Some examples of such ligands are

M      Basic Terms and Types of Ligands Class 12 Notes | EduRev NO2-            M      Basic Terms and Types of Ligands Class 12 Notes | EduRev ONO-
                nitrite - N          nitrite -O

M Basic Terms and Types of Ligands Class 12 Notes | EduRev CN-                   M Basic Terms and Types of Ligands Class 12 Notes | EduRev NC-
           cyanide             isocyanide

M Basic Terms and Types of Ligands Class 12 Notes | EduRev SCN-                M Basic Terms and Types of Ligands Class 12 Notes | EduRev NCS-
         thiocyanide        isothiocyanide

(Viii) Ligands having more than two donor atoms are called polydentate or multidentate ligands. Multidentate ligands are known as a chelating ligands, it results in the formation of a stable cyclic ring thus, the complexes formed are also called chelates. Chelating ligands are usually organic compounds.

3. Co-ordination sphere The central metal atom and the ligands directly attached to it are collectively termed as the co-ordination sphere. Co-ordination sphere is written inside square brackets, for examples, [Co(NH3)6]3+ . Remember that the central metal atom and the ligands inside the square brackets, behave as a single entity.


Basic Terms and Types of Ligands Class 12 Notes | EduRev

Fig: Co-ordinate Sphere


4. Co-ordination number (CN) The co-ordination number (CN) of a metal atom /ion in a complex is the total number of e- pairs accepted by central metal atom /ion from ligands through coordinate bond. Some common co-ordination numbers of metal ions are summarised in the following Table (1) and examples of complexes of various co-ordination number are given in Table (2).
Table (1) Co-ordination number of metal ion 

Metal ion

Co-ordination number

Ag+

2,4

Cu+

2,4

Cu2+

4,6

Au+

2,4

Ca2+

4,6

Fe2+

4,6

Fe3+

6

Co2+

4,6

Co+3

6

Ni2+

4,6

Zn2+

4

Al3+

4,6

Sc3+

6

Cr3+

6

Pd2+,Pt2+

4

Pd4+, Pt4+

6

Table (2) Examples of complexes of various co-ordination numbers 

Basic Terms and Types of Ligands Class 12 Notes | EduRev 

5. Oxidation number/oxidation state (O.S.) of central metal ion It is a number(numerical value) which represents the electric charge on the central metal atom of a complex ion. for example the oxidation number of Fe, CO and Ni in [Fe(CN)6]4-, [Co(NH3)6] and Ni(CO)are 2, 3 and zero, respectively. Let us take a few examples to illustrate this.

(i) Potassium Ferrocyanide, K4[Fe(CN)6] Since the complex has four monovalent cations outside the coordination sphere, the complex ion must carry four negative charges, i.e., it is [Fe(CN)6]4-. The number of CN- ion (univalent ion), that is 6 represents the co-ordination number of Fe cation. The oxidation state of iron can be determined easily as below, knowing that cyanids ions is unidentate and the complex on the whole carries -4 charge.

[Fe(CN)6]-4

x + (-6) = -4

Therefore, x = 2

Thus, here iron is present as Fe2+  or Fe(II).

(ii) [Cr(C2O4)3]3- Note that here the oxalate ligand is denegative ion, that is it is bidentate. Therefore three oxalate ligands carry a total charges of -6 and co-ordination number of Cr is 6. Now since the complex carries -3 charge, therefore the oxidation state of Cr is 3.

(iii) Ni(CO)4 Here the co-ordination number of Ni is 4 since carbonyl group is unidentate. Further since the complex as well as the ligands has no charge, nickel atom must also be neutral ,that is it is in zero oxidation state.

6. Effective atomic number - EAN (Sidgwick Theory and EAN Rule) : Total no. of electrons present on central metal atom /ion. after accepting electron pairs from donar atom of ligands through coordinate bond is called E.A.N. of central metal atom /ion.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Sidgwick also suggested that the metal ion will continue accepting electron pairs till the total number of electrons in the metal ion and those donated by ligands is equal to that

of nearest noble gas. This total number of electrons is called effective atomic number (EAN) of the metal /ion. This will become clear by taking the example of hexamminecobalt (III) ion [Co(NH3)6]

Atomic number of cobalt = 27

In the present complex, cobalt is present in the oxidation state of 3.

Therefore, E.A.N. of Co3+  = Z - O.S. + 2 × C.N.

= 27 - 3 + 2 × 6 = 36

In the above example since the number 36 corresponds to the atomic number of krypton, according to Sidgwick, the complex will be stable. Though EAN rule (which states that those complexes are stable whose EAN is the same as the atomic number of the next noble gas) is applicable in many metal carbonyl complexes, however there are several examples in which EAN rule is not obeyed.

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