Basic Terms and Types of Ligands Class 12 Notes | EduRev

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

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Introduction: Werner’s Theory

Alfred Werner in 1898 proposed Werner’s theory explaining the structure of coordination compounds.

Werner’s Experiment: By mixing AgNO3 (silver nitrate) with CoCl3·6NH3, all three chloride ions got converted to AgCl (silver chloride). However, when AgNO3 was mixed with CoCl3·5NH3, two moles of AgCl were formed. Further, on mixing CoCl3·4NH3 with AgNO3, one mole of AgCl was formed. 


Postulates of Werner’s Theory

Based on this observation, the following Werner’s theory was postulated:

  • The central metal atom in the coordination compound exhibits two types of valency, namely, primary and secondary linkages or valencies.
  • Primary linkages are ionizable and are satisfied by the negative ions.
  • Secondary linkages are non-ionizable. These are satisfied by negative ions. Also, the secondary valence is fixed for any metal and is equal to its coordination number.
  • The ions bounded by the secondary linkages to the metal exhibit characteristic spatial arrangements corresponding to different coordination numbers.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

Difference between Primary and Secondary Valency in Coordination Compounds:

Basic Terms and Types of Ligands Class 12 Notes | EduRev


Limitations of Werner’s Theory

  • It fails to explain the magnetic, colour and optical properties shown by coordination compounds.
  • It failed to explain the reason why all elements don’t form coordination compounds.
  • It failed to explain the directional properties of bonds in coordination compounds.
  • This theory does not explain the stability of the complex
  • This theory could not explain the nature of complexes

Try yourself:What was the term proposed by Werner for the number of groups bound directly to the metal ion in a coordination complex?
View Solution

Basic Concepts/Terms:

Coordination Entity

A chemical compound in which the central ion or atom (or the coordination centre) is bound to a set number of atoms, molecules, or ions is called a coordination entity.

Examples: [CoCl3(NH3)3], and [Fe(CN)6]4-.


Central Atoms and Central Ions

The atoms and ions to which a set number of atoms, molecules, or ions are bound are referred to as the central atoms and the central ions. In coordination compounds, the central atoms or ions are typically Lewis Acids and can, therefore, act as electron-pair acceptors.


Ligands

The atoms, molecules, or ions that are bound to the coordination centre or the central atom/ion are referred to as ligands. These ligands can either be a simple ion or molecule (such as Cl or NH3) or in the form of relatively large molecules, such as ethane-1,2-diamine (NH2-CH2-CH2-NH2).


Coordination Number

The coordination number of the central atom in the coordination compound refers to the total number of sigma bonds through which the ligands are bound to the coordination centre.

Example: [Co(NH3)6]3+, which features a 6-coordinate metal centre with octahedral molecular geometry, have coordination number 6

Basic Terms and Types of Ligands Class 12 Notes | EduRev


Coordination Sphere

The non-ionizable part of a complex compound consists of a central transition metal ion surrounded by neighbouring atoms or groups enclosed in a square bracket.

The coordination centre, the ligands attached to the coordination centre, and the net charge of the chemical compound as a whole, form the coordination sphere when written together. This coordination sphere is usually accompanied by a counter ion (the ionizable groups that attach to charged coordination complexes).

Example: In the complex K4[Fe(CN)6], [Fe(CN)6]4– is the coordination sphere and K+ is the counter ion.


Coordination Polyhedron

The geometric shape formed by the attachment of the ligands to the coordination centre is called the coordination polyhedron.

Example: The most common coordination polyhedra are octahedral, square planar and tetrahedral. For example, [Co(NH3)6]3+ is octahedral, [Ni(Co)4] is tetrahedral and [PtCl4]2– is square planar. 


Oxidation Number

The oxidation number of the central atom can be calculated by finding the charge associated with it when all the electron pairs that are donated by the ligands are removed from it.

Example: The oxidation number of the platinum atom in the complex [PtCl6]2- is +4.


Homoleptic and Heteroleptic Complex

Homoleptic Complex: When the coordination centre is bound to only one type of electron pair donating ligand group, the coordination complex is called a homoleptic complex, for example: [Cu(CN)4]3-.

Heteroleptic Complex: When the central atom is bound to many different types of ligands, the coordination compound in question is called a heteroleptic complex, an example for which is [Co(NH3)4Cl2]+.


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 who lose 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

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.

2. Ligands: Species that 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 that have at least one lone pair.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

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

Basic Terms and Types of Ligands Class 12 Notes | EduRevLigands

Ligands can be of the 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 | EduRevUnidentate 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

diethylenetriamine (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 the ethylenediaminetetraacetate ion.

Basic Terms and Types of Ligands Class 12 Notes | EduRev

(vii) Ambidentate ligands: There are certain ligands that 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

Basic Terms and Types of Ligands Class 12 Notes | EduRev


(viii) Ligands having more than two donor atoms are called polydentate or multidentate ligands. Multidentate ligands are known as a chelating ligand, 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 coordination sphere. The coordination 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


4. Co-ordination number (CN) The coordination 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 the coordinate bond.
Basic Terms and Types of Ligands Class 12 Notes | EduRev


Basic Terms and Types of Ligands Class 12 Notes | EduRev

Examples of complexes of various co-ordination numbers

Try yourself:Which of the following is a complex salt?
View Solution

5. Oxidation number/oxidation state (O.S.) of central metal ion It is a number(numerical value) that represents the electric charge on the central metal atom of a complexion. 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 coordination number of Fe cation. The oxidation state of iron can be determined easily as below, knowing that cyanides ions are unidentate and the complex, on the whole, carries a -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 a negative ion, that is it is bidentate. Therefore three oxalate ligands carry a total charge of -6 and the coordination number of Cr is 6. Now since the complex carries a -3 charge, therefore the oxidation state of Cr is 3.

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

Try yourself:What is the sum of the oxidation number of cobalt in [Co(H2O)(CN)(en)2]2+and [CoBr2(en)2]+?
View Solution

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 donor atom of ligands through coordinate bond is called E.A.N. of central metal atom /ion.

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 a nearest noble gas. This total number of electrons is called the effective atomic number (EAN) of the metal /ion. 

This will become clear by taking the example of hexamine cobalt (III) ion [Co(NH3)6]

The 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 the 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 the EAN rule is not obeyed.

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