Important terminology of coordination chemistry

 Important terminology of coordination chemistry - 

Coordination Entity- Constitutes a central metal atom or ion bonded to a fixed number of ions/ molecules.it is written in square bracket. Example 

[Ni (CO)4] 

[PtCl2(NH3)2] 

[Fe (CN)6]4+ 

 b). Central atom/ion- in coordination entity, the atom/ion to which a fixed          no. of ions/group are bound in a definite geometrical arrangement around it.  Example

[Ni (CO)4]             Ni0                 

[PtCl2(NH3)2]       Pt2+

[Fe (CN)6]4+        Fe2+ 

c). Ligands – The neutral molecules or anions or cations which are directly linked with the central metal atom or ion in a complex ion are called ligands. 

The ligands are attached to the central ion or atom through coordinate bonds or dative linkage. 

Types of ligands-  

Ligands can be of following types depending on the number of donor atoms or no of donor site present in them. 

Mono- or unidentate ligands: They have one donor atom, i.e., they supply only one electron pair to central metal atom or ion. F1-, Cl1-, H2O , NH3 etc. 

Bidentate ligands: Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. 

Example- 

 H2NCH2CH2NH2  (ethane-1,2-diamine) 

 C2O42-.   oxalate 

Polydentate ligands- having donor site/atom more than two. 

Example – 

[EDTA4-]  Ethylenediaminetriacetate 

(EDTA3-)   Ethylenediaminetriacetate 

Ambidentate ligands- ligand which has two donor site but use one to bind a metal atom in a time is called ambidentate ligand

Example- NO₂^- ,CN^1- , SCN^1- etc

M←CN    ,     M→NC          

here M is metal atom /ion

 and CN^-   -ambidentate ligand     

Chelating ligands:  

A bidentate or a poly dentate ligand uses its two or more donor atoms/site to bind the same metal ion simultaneously and form one or more rings are called chelate or chelating ligands 

The formation of ring is called chelation. 

No of ligating of di or polydentate ligand is called denticity. 

Chelating ligands form more stable complexes than monodentate ligands. This is called chelating effect. All types of polydentate ligands can act as chelating ligands.

•               most stable                                           less stable

• 
  

•       chelate molecule                                          Non-Chelate molecule

•        

• d).  Coordination number – 

• The number of atoms of the ligands that are directly bound to the central metal atom or ion by coordinate bonds is known as the coordination number of the metal atom or ion. 

• Example                      coordination no.

• [Ni (CO)4]        —     4                      

• [PtCl2(NH3)2]  —     4 

• [Fe (CN)6]4+    —     6 

 

e). Coordination sphere or Coordination entity and counter ions- 

• The central metal atom or ion and the ligands that are directly attached to it are enclosed in a square bracket. It has been called coordination sphere or coordination entity. 

• The ionizable groups written outside the bracket are called counter ions. 

   4K [Fe (CN)6] -----> [Fe (CN)6]4-   +     4K+ 

                              coordination sphere      counter ions. 

f). oxidation number of central atoms is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. 

                                              Oxidation no. 

 [Ni (CO)4]                      Ni0              0

[PtCl2(NH3)2]              Pt2+            0

[Fe (CN)6]4+                 Fe2+            0 

 

g). Homoleptic complex- Complex in which a metal is bound to only one kind of donor group is called homoleptic complex. Example- [Ni (CO)4]    

h). Heteroleptic complex   - Complex in which a metal is bound to only one kind of donor group is called homoleptic complex. Example-      [PtCl2(NH3)2]  

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Werner theory of coordination compound

 

Werner theory of coordination compound

·      Proposed by Alfred Werner in 1893

·       He prepared and characterised a large no of coordination compounds and studied their physical and chemical properties

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

      1mole   CoCl₃.6NH₃ (yellow) gave à 3mole AgCl

      1mole   CoCl₃.5NH₃ (purple) gave à2mole AgCl

      1mole   CoCl₃.4NH₃ (green₎ gave à1mole AgCl

      1mole   CoCl₃.4NH₃(violet) gave à1mole AgCl

The important postulates of coordination theory are as –

1). in coordination compound, metals show types of valencies/linkage.

      a). Primary valencies

      b). secondary valencies

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

3). Secondary valencies are non-ionisable and satisfied by neutral molecules or negative    

         Ions. It represents to the coordination number and is fixed for a metal.

4. the ions/group of atoms bound by the secondary linkage to the metal have characteristics spatial arrangement corresponding to different coordination number.

 

In modern formulation –

·      Spatial arrangement of ions/ groups present around to central metal ion is called coordination polyhedral.

·      The species within the square bracket are coordination entities or complex

·      Ions outside the square bracket are called counter ions.

·      Primary valencies usually indicates to the oxidation no. and Secondary valencies refers to the coordination no. of central metal atom/ions.