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(NH3)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].
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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