Coordination Compounds : General terms

Double Salts and Co-ordination Compounds.

When solutions of two or more stable compounds are mixed in stoichiometric (simple molecular) proportions new crystalline compounds called molecular or addition compounds are formed. These are of two types :

(1)     Double salts,     

(2)     Co-ordination or Complex compounds

 

(1)     Double salts : Addition compounds, stable in solid state. Dissociate into ions in aqueous solution as such give test for each constituent ion. Examples:

 

Double Salt Responds test for the ions
Carnalite :KCl. MgCl26H2O K+, Mg2+, Cl–
Potash alum  :

K2SO4. Al2(SO4)3 24H2O

 K+, Al3+, SO42–

                  

(2)     Co-ordination or Complex Compounds : Addition compound, stable in solid state.  Retain their identity even in solution.  Central metal ion form dative or coordinate bond with the species surrounding it (ligands). Examples :

 

Complex compound Cation Anion
[Cu(NH3)4SO4 [Cu(NH3)4]+2 SO42–
K2[PtF6] 2K+ [PtF6]2–
[Co(NH3)6] [Cr(CN3)6] [Co(NH3)6]2+ [Cr(CN)6]3–

 

Terminology of co-ordination compounds.

(1)     Central metal atom or ion : A complex ion contains a metal atom or ion known as the central metal atom or ion. It is sometimes also called a nuclear atom.

(2)     Complex ion : It is an electrically charged radical which is formed by the combination of a simple cation with one or more neutral molecules or simple anions or in some cases positive groups also.

(3)     Ligands : Neutral molecules or ions that attach to central metal ion are called ligands. The donor atom associated with the ligands supplies lone pair of electrons to the central metal atom (forming dative bond) may be one or two more. Monodentate (one donor atom), bidentate (two donor atom), tridenatate (three donor atom) etc.

Monodentate Ligands (with one donor site)

 

Anionic Ligands (Negative legands)

Formula Name Formula Name
X– Halo O22– Peroxo
:OH– Hydroxo CH3COO– Acetato
CN– Cyano NO3– Nitrato
O2– Oxo S2O32– Thiosulphato
NH2– Amido NO2– Nitrito
S2– Sulphido CO32– Carbonato
CNS– Thiocyanato SO42– Sulphato

 

Neutral Ligands

Formula Name Formula Name
CO Carbonyl : NH3 Amminato
PH3 Phosphine H2O Aqua
NO Nitrosyl C6H5N : Pyridine (py)

 

Cationic  Ligand (Positive)

Formula Name Formula Name
NO2+ Nitronium NO+ Nitrosonium
H2NNH3+ Hydrazinium  

                  

Polydentate ligands (with two or more donor site)

Bidentate (Two donor sites)

Formula Name Formula

Name
H2NCH2CH2NH2 Ethylenediamine (en)   Dimethylglyoximeto (dmg)
  Oxalato (ox) NH2–CH2

–COO–

 Glycinate ion (gly) 


  Formula Name
Tridentate   { H }_{ 2 }\ddot { N } (C{ H }_{ 2 })_{ 2 }-\ddot { N } H(C{ H }_{ 2 })_{ 2 }\ddot { N } H_{ 2 } Diethylenetriaminediamine (dien)
Tetradentate { H }_{ 2 }\ddot { N } (C{ H }_{ 2 })_{ 2 }-\ddot { N } H(C{ H }_{ 2 })_{ 2 }   Triethylenetetramine (trien)
Hexadentate   Ethylenediamine tetra acetic acid (EDTA)4–

 

Chelating Ligand : When polydentate ligands bind to the central metal ion they form a ring called chelate and the ligand is referred as chelating ligand.

Ambidentate ligands :  A ligand which possesses two donor atom but in forming complex it utilizes only one atom depending upon the condition and type of complex.

 (nitro) , ONO (nitrito),  CN (cyano),  NC (isocyano), SCN (thiocyanide), NCS (isothiocyanide)

π- acid ligand : Ligands which are capable of accepting an appreciable amount of π- e– density from the metal atom into emptying π or π* orbital or their own called π– acceptor or π– acid ligands eg. CO.

(4)     Co-ordination Sphere : Ligand with central metal ion is kept in square bracket [ ] retains its identity in the same form is called co-ordination sphere (non-ionisable)

(5)     Co – ordination Number : Number of monodentate ligands attached to central atom/ ion are called coordination number of the central metal atom/ion.

(6)     Ionisation Sphere : The part present out side of the square bracket is called ionization sphere (ionisable).