Coordination compound Notes

Coordination Compound

Coordination Compound
A coordination or complex compound may be defined as a compound formed by the combination of two or more simple stable compounds and do not loose its identity in solid as well as in liquid phase.
Example : [Cu(NH₃)₄]SO₄

Double Salts or Lattice Compounds
The addition compound which are stable in solid state only but are broken down into parts when dissolved in water are called Double Salts.
Example :
FeSO₄.(NH₄)₂SO₄.6H₂O ------>> Fe²⁺_(aq) + 2NH₄⁺_(aq) + 2SO₄^2-_(aq) + 6H₂O
FeSO₄.(NH₄)₂SO₄.6H₂O = Mohr Salt

Difference between Double Salt and Coordination Compound

	Coordination Compound	Double Salts
1.	It exists in the solid state as well as in water or any other solvent.	It exists only in solid phase.
2.	Same ion present in the constituents are not obtained when dissolved in water.	The ion present in the constituents are obtained when dissolved in water.
3.	It does not lose its identity in solution..	It lose its identity in solution.
4.	It contain ionic as well as coordinate bonds.	It contain ionic bonds. Metal shows their normal valancy.
5.	The properties of a coordination compound are usually different from those of the constituent compounds.	The properties of the double salt are nearly the same as those of the constituent compounds.

Terminology of Coordination Compounds
Ligands
The neutral molecules, anions or cations which are directly linked with the central metal atom or ion in a complex ion, are called Ligands.
Ligands acts as a donar as it donates one or more electron pair to the central metal ion or atom which act as an acceptor.
-- Condition for formation of complex ion --
1. Ligands should have lone pair or pairs of electron which can be donated to the central atom or ion.
2. The central metal atom or ion should have vacant orbitals of almost same energy.

Type of Ligands :
1. Monodentate or Unidentate Ligands :
They have one donar atom, i.e. they supply only one electron pair to central metal ion or atom.
Example : F^-, Cl^-, Br^-, H₂O, NH₃ etc.

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2. Polydentate Ligands :
They have one or more donar atom, i.e. they have to ability to link with central atom ion at two or more position.
Example :

Ethylenediamine (en) (bidentate)
2,2',2''-Terpyridine (terpy) (tridentate)

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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 Coordination Number.
Examples :

[Ag(CN)2]-	Coordi. No. of Ag+ = 2 (as (CN)2 give 2 electron pair i.e. 2 lone pairs to Ag.)
[Fe(C2O4)3]3-	Coordi. No. of Fe3+ = 6 (as (C2O4) is bidentate so (C2O4)3 will give 6 electron pairs to Fe)
[PtCl6]2-	Coordi. No. of Pt4+ = 6 (Cl is monodentate)

Note :- 3 membered ring is not possible in complex so in (CN^-) only one can donate its lone pair. (either C or N)
This is due to increase in electron density in small region which increase repulsion.

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. This has been called coordination sphere or first sphere of attraction or Coordination entity. It behaves as a single unit because the ligands present in the coordination sphere are held tightly by the metal ion.
Any ion present outside this sphere is separated from the complex when the compound is dissolved in the water, or any other polar medium. The ion present outside the coordination sphere are called counter ion.

Charge on the complex ion :
The charge on the complex ion is the algebraic sum of the charges carried by central atom or ion and the ligands attached to it.

Oxidation number or oxidation state of central atom
It is defined as the charge which the central atom would carry if all the ligands are removed along with the electron pairs from the coordination sphere.
Charge on the complex ion = Oxidation no. of metal ion + Charge on ligands

Examples : --- Find X ---

[Hg(CN)4]X

The oxidation no. of Hg in the complex is +2. X= +2 + 4*(-1) = -2, i.e. [Hg(CN)4]2-

-- Classification of Coordination Compound --
1. On the basis of Charge --
a)Neutral Complex
Those complex whose sum of total charge is zero, are Neutral Complex.
Examples : [Ni(CO)₄]⁰, [Fe(CO)₅]⁰
b)Ionic Complex
Those complex whose sum of total charge is either negative or positive, are Ionic Complex.
Examples :
K₄[Fe(CN)₆]^4-, K₃[Fe(CN)₆]^3- --- anionic complex
[Pt(NH₃)₆]⁴⁺Cl₄, [Ag(NH₃)₂]⁺Cl --- cationic complex
[Cr(NH₃)₆]³⁺[Co(CN)₆]^3-, [Pt(NH₃)₄]²⁺[CuCl)₄]^2- --- cationic and anionic complex

1. On the basis of Ligands --
a) Homoleptic Complex : --
If all ligands are same in complex.
Examples : K₄[Fe(CN)₆]
b) Heteroleptic Complex : --
If different ligands are present in complex.
Examples : [Fe(H₂O)₄ClBr]NO₂

In Coordination Complex, central metal atom shows two type of Valancy -->

1. Primary Valancy :--
It is equal to the oxidation number of the central atom.
Examples :

K4[Fe(CN)6]

Oxidation no. of K is +1. so K4= +4 , Fe = X , CN- has -1 charge. so (CN-)6 has -6 charge. +4 + X + (-6) = 0 X = +2.

2. Secondary Valancy / Coordination Number / Auxiliary Number :--
It is equal to the no. of loan pair accepted by central metal atom from Ligands.

Complex	Primary Valancy	No. of Ligands	Coordination No. / Secondary Valency
K4[Fe(CN)6]	+2	6	6
[Fe(en)3]+3	+3 (as 'en' is neutral)	3	6

==> Coordination no. and no. of ligands may or may not be equal.
==> Coordination no. 2, 4, 6 is more common, 3, 5, 7 is less common and more than 7 is rarely possible in complex.
==> Coordination no. depends on charge, size of central atom and ligands.

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Metal	Metal charge	Coordination Number
Ag	+1	2
Cu	+1
Au	+1
Fe	+2	4 or 6
Co	+2
Ni	+2
Cu	+2
Mn	+2
Fe	+3	6
Co	+3
Ni	+3
Cu	+3
Cr	+3
Sn	+2	4
Pb	+2
Pd	+2
Pt	+2
Zn	+2
Cd	+2
Hg	+2
Sn	+4	6
Pb	+4
Pd	+4
Pt	+4
Au	+3	4
Al	+3	4 or 6

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Chelating Ligands

1. All polydentate are Chelating ligands because they form a ring with central atom.
   
2. Three membered ring is not allowed.

Examples :

1.	[M(en)] no. of 5-membered ring = 1
2.	[Fe(en)3]+3 no. of 5-membered ring = 3
3.	[M(CN-)]

Members	Structure
2 - members form	linear
3 - members form	triangular
4 - members form	tetrahedral
5 - members form	triangular pyramidal
6 - members form	triangular bipyramidal or octahedral

[M(edta)] Total angle between N M O is 8 in which 6 are 90o and 2 are 180o.

Ques. 1. H₂O₂ and N₂H₄ cann't act as bidentate ligand. Explain?

Ans. Both not exist as bidentate ligand because they form three membered ring which is not possible to form.

Ques. 2. CN^- and SCN^- cann't act as bidentate ligand. Explain?

Ans. " C M N " is three membered ring (not possible). And sp hybridised atom is not possible in 4 membered ring because angle between S C N is 180, which increase the bond length between S --- M and N --- M. So bond between them will not form. It is 4 - membered ring but due to increase in bond length, it is not possible to form.

Ambidentate Ligands
It has two or more different donar side but only one of them is used or donate its lone pair.
Examples :

NC- ---> C-N --->	CO ---> OC --->
NCO- ---> OCN- --->	NCS- ---> S-CN --->
ONO- ---> O-ON --->	etc.

Flexidentate Ligands
All the polydentate ligands are flexidentate because they can change their dentisity as per requirement of central atom.
Examples :
Fe⁺³ = Coordination number = 6
a] [Fe(H₂O)₆]⁺³.
b] [Fe(en)₃]⁺³.
c] [Fe(H₂O)₄(SO₄)]⁺.
d] [Fe(H₂O)₅(SO₄)]⁺.
In compound 'c', (SO₄)^2- act as bidentate ligand but In compound 'd', it act as monodentate ligand. (SO₄)^2- adjust its dentisity as per requirement of central atom. So it is a flexidentate ligand.

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