Kingdom Protista

Co-ordination Compound

                                 

            Co-ordination Compound

Werner’s Theory :   Postulates of this theory are –    

ØEach metal in co-ordination compound possesses two type of valencies :

i)Principal valency or Primary valency or ionisable valency.

ii)Secondary valency or non-ionisable valency.

Ø Primary valencies are satisfied by anion only. The number of primary valencies depends upon the oxidation state of the central metal.These are represented by dotted lines between central metal atom and anion.

Ø  Secondary valencies are satisfied only by electron pair donor, the ions or the neutral species. These are represented by thick lines.

Ø  Every central ions tends to satisfy its primary as well as secondary valencies.

Ø  The secondary valencies are directional and are directed in space about the central metal ions. The primary valencies are non-directional.

Ø  The presence of secondary valencies gives rise to stereoisomerism in complexes.

Ø  The ions attached to the primary valency possesses ionizing nature whereas, the ions attached to secondary valencies do not ionizes when the complex is dissolved in a solvent.

Ø   Initially, Werner had pointed out co-ordination number of a metal to be four or six.

Note: Now it has been proposed that coordination number of a metal may be any whole number between 2 and 9.

 Double salt :    A double salt is a combination of two or more than two stable compound in a stoichiometry ratio. Double salt get completely ionized in water.

Exam : Potash alum (K₂SO₄ . Al₂ (SO₄)₃ .24H₂O)

Co-ordinate compound :    The compounds which does not split up into simple ions completely when dissolved in water.

Exam : K₄ [Fe(CN)₆]. When this compound is dissolved in water then it breaks up into 4K⁺ and [Fe(CN)₆] but the co-ordination entity or species written inside the square bracket does not get ionized.

Co-ordination Entity :   It constitute a central metal ion or atom along with ligands.

The species written inside the square bracket are called as co-ordination entity.

Exam:  [Fe(CN)₆]^-4_.

Ligands :  These are the ions or molecules which are bounds to the central metal atom or the ions in the co-ordination entity. They act as donor species. So they can be negatively charged ion or electron rich neutral molecules.

 Exam : Cl^-, OH^-, NH₃, H₂O etc.

Denticity :   It is the number of donor atom which bound a central metal atom or ion in a co-ordination entity. Depending on denticity ligands can be –

1)Unidentate :    When a ligand is bound to a central metal ion through a single donor atom then it is called as unidentate ligands.

Exam : Cl^-, OH^-, NH₃, H₂O etc

2)Bi-dentate ligands :   When a ligand bounds a central metal ion through two donor atom, then it is said to be bi-dentate ligands.

Exam : ^-OOC – COO^-.

3)Poly-dentate ligands :  When a ligand bounds a central metal ion through more than two donor atom, then it is called as poly-dentate ligands.

Exam : EDTA (Ethylene Di-amine Tetra Acetate). The denticity of EDTA is 6.

Co-ordination number : It is the no. of ligands donor atom which bounds the central metal ion or atom.

Exam : K₄ [Fe(CN)₆]           = 6

              [Co Cl₃ (NH₃)₃]     = 6

Homoleptic complexes :   If the co-ordination complexes  have a single kind of ligands or donor group which bounds central metal atom or ion, then it is called as Homoleptic complexes.

Exam : [Fe(CN)₆]⁺³_.

Heteroleptic complexes :   If the coordination complex has more than one type of donor group or ligand which bounds the central metal ion or atom, then it is called as heteroleptic complexes.

Exam : [Fe(CN)₃ Cl₃]⁺³.

Name of ligands

Symbol of ligand

Ø  Fluorido Ø  Chlorido Ø  Bromido Ø  Iodido Ø  Oxalate Ø  Aqua Ø  Ammine Ø  Ethane -1,2-diamine Ø  Cyano Ø  ISocyano Ø  Nitrito-N Ø  Nitrito- O Ø  Sulphato Ø  Triphenyl phosphine Ø  Methyl amine Ø  Hydroxo Ø  Carbonyl Ø  Thio cyanato Ø  Nitrato Ø  Iso thio cyanato Ø  Carbonato Ø  Pyridine Ø Acetato Ø  Nitrato

F-                  Cl-                   Br-                   I-                  (C2O4)-2 or OX                   H2O                   NH3                   NH2CH2CH2NH2                   CN-                    NC-                    NO2-                    ONO-                    SO4-2                    PPh3                    CH3NH2                     OH-                     CO                     CNS-                     NO3-                     NCS-                     CO3-2                      Py                      CH3COO—                                NO3-

Rules for Naming Co-ordination Compounds :

Ø Co-ordination compounds are named from left to right in a following sequence –

Name of left counter ion (without mentioning its no. if present)     

           Name of ligands along with co-ordination number [if more than one ligand is present then alphabetical order will be follow]

                      Name of central metal atom/ion 

                  Oxidation of the central metal atom/ion in roman number

        

   Name of right counter ion [without mentioning its number if present]

Ø If co-ordination complex is negatively charged or anionic then suffix ‘ate’ is added to the name of metal.

     Exam :

Name of metal	Name used in compound
Ø  Zn Ø  Fe Ø  Cu Ø  Mn Ø  Ag Ø  Au Ø  Pd Ø  Pt Ø  Hg	Zincate         Ferrate          Cuperate          Manganate          Argentate          Aurrate          Palladate          Platinate          Mercurate

Ø If the complex is positively or negatively charged, without any counter ion then after oxi. state ‘ion’ is added.

Ø If there are more than one kind of ligand, then they are named according to their alphabetical order.

Ø If a ligand has a numeric prefix like di, tri etc. in its name, then prefix bis or tris are used to indicate its number of molecules.

Exam:    1)   K₄[Fe(CN)₆]

                            Potassium hexa cyano ferrate(II)

                2) [Fe(CN)₆]^-4

                            Hexa cyano ferrate (II) ion.

                3) [Fe(H₂O)]Cl₃

                            Hexa aqua iron (III) chloride

  

Isomerism in Co-ordination compound :

Ø Structural Isomerism :

      The compounds which have same molecular formula but different structural formula are called as structural isomers.

Structural isomers are of four type –

1)Ionisation isomerism : This form of isomerism arises when the counter ion in the complex salt is itself a potential ligand and can be displace a ligand which can becomes the counter ion.

 

Exam : [Co(NH₃)₅ Br]SO₄   and  [Co(NH_3­)₄SO₄)NH₃Br are ionisable isomers.

2) Hydrate/solvent isomerism : This form of isomerism arises when a counter ion changes its position with H₂O acting as ligand.

 

Exam : [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂H₂O are hydrate isomers.

3) Linkage isomerism : This type of isomerism arises in a co-ordination compound containing Ambidentate ligands like NO₂^-, CN^-, SCN^- etc.

Exam:  [Co(NH₃)₅NO₂]Cl₂ and [Co(NH₃)₅ONO]Cl₂.

4) Co-ordinate isomerism : This type of isomer arises from the interchange of ligands between two co-ordination entities.

Exam : [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

 Valence bond theory :  

The salient feature of this theory are-

• The central metal ion has a number of empty orbital for accommodating electrons donated by the ligands. The number of empty orbital is equal to the co-ordination number of the metal ion for the particular complex.
• The atomic orbital (s, p or d) of a metal ion hybridize to form hybrid orbital with definite directional properties. These hybrid orbital now overlaps with the ligands orbital to form strong chemical bonds.
•  The d-orbital involved in the hybridization may be either inner (n-1)d or outer n-d orbital. The complexes formed in the two ways are referred to as low  spin or high spin complexes, respectively.
• Each ligand contains a lone pair of electrons.
• A covalent bond is formed by the overlapping of a vacant hybridized metal orbital and a filled orbital of a ligand. The bond is also sometime called as co-ordinate bond.
•  If a complex contains an unpaired electrons, it is paramagnetic in nature, while if it does not contains unpaired electron, it is diamagnetic in nature.
• The number of unpaired electrons in the complex, points out the geometry of the complex as well as hybridization of the central metal ion and vice-versa. In practice, the number of unpaired electrons in a complex as formed from magnetic moment measured from-

                                       μ={n(n+2)}^1/2

                     {where n= no. of lone pair of electrons.}

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