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Coordination Compounds
Double salts dissociate completely into simple ions when dissolved in water.
However, complex ions such as [Fe (CN) 6]4- of K4 [Fe (CN) 6], do not
dissociate into Fe2+ and (CN) 1- ions. Mohr’s salt and Potash alum are double
salts
Main Postulates of Werner’s Theory: i) Metals show two types of valences in
coordination compounds- Primary & Secondary. ii) The Primary valences are
normally ionizable and are satisfied by negative ions. iii) The Secondary
valences are non ionizable and are satisfied by neutral molecules or negative
ions (Ligands). iv) The ions/groups bound by the secondary linkages to the
metal have characteristics spatial arrangements (coordination polyhedra)
corresponding to different coordination numbers
A coordination entity comprises of metal atom or ion bounded with fixed no.
of molecules or ions. Ex. K4 [Fe (CN) 6]
Ions or molecules bound to the central metal atom or ion is known as ligand.
When a ligand is bound to a metal ion through a single donor atom, as with
Cl–, H2O or NH3, the ligand is said to be unidentate. When a ligand can bind
through two donor atoms as in H2NCH2CH2NH2 (ethane-1,2-diamine) C2O4 2–
(oxalate), the ligand is said to be didentate and when several donor atoms are
present in a single ligand as in N(CH2CH2NH2)3, the ligand is said to be
polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is an important
hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to
a central metal ion.
The number of coordinating or ligating groups present in a ligand is called the
denticity of that ligand. When a di- or polydentate ligand uses its two or more
donor atoms to bind a single metal ion, it is said to be a chelate ligand.
Ligand which can ligate through two different atoms is called ambidentate
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ligand (e.g. NO2 and SCN ions).
The coordination number of a metal ion in a complex can be defined as the
number of ligand donor atoms to which the metal is directly bonded. The
coordination number of the complex ions, [PtCl6] 2-, [Ni(NH3) 4] 2+,
[Co (en) 3] 3+ and [Fe (C2O4)3] 3- are 6, 4, 6 and 6 respectively.
It is important to note that the coordination number of the central atom/ion is
determined only by the number of sigma bonds formed by the ligand with the
central atom/ion. Pi bonds, if formed between the ligand and the central
atom/ion, are not counted for this purpose.
Complexes in which a metal is bound to only one kind of donor groups are
known as homoleptic complexes.e.g. [Co (NH3)6] 3+. Complexes in which a
metal is bound to more than one kind of donor groups are known as
heteroleptic complexes. e.g. [Co (NH3) 4 Cl2] +.
Chelate effect: When a polydentate ligand forms a coordination complex
having a ring structure, it is called chelate complex. A chelate complex is
always more stable than the complex formed by monodentate ligands. The
stabilization of coordination compound due to chelation is called chelate
effect.
The coordination number (CN) of a metal ion in a complex can be defined as
the number of ligand donor atoms to which the metal is directly bonded. For
example, in the complex ions, [PtCl6]2– and [Ni(NH3)4]2+, the coordination
number of Pt and Ni are 6 and 4 respectively. Similarly, in the complex ions,
[Fe(C2O4)3]3– and [Co(en)3]3+, the coordination number of both, Fe and Co, is
6 because C2O4 2– and en (ethane-1,2-diamine) are didentate ligands.
The central atom/ion and the ligands attached to it are enclosed in square
bracket and is collectively termed as the coordination sphere.
The spatial arrangement of the ligand atoms which are directly attached to the
central atom/ion defines a coordination polyhedron about the central atom.
The most common coordination polyhedra are octahedral, square planar and
tetrahedral. For example, [Co(NH3)6]3+ is octahedral, [Ni(CO)4] is
tetrahedral and [PtCl4]2– is square planar.
IUPAC NAMES OF SOME COORDINATION COMPOUNDS
1. [Co (NH3)4(H2O) 2] Cl3 = Tetraamminediaquacobalt(III) chloride
2. [Cr(en)3] Cl3 = Tris(ethane-1,2-diamine)chromium(III) chloride
3.
4.
5.
6.
7.
8.
9.
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[Pt(NH3)BrCl(NO2)] = Amminebromidochloridonitrito-N-platinate(II)
[PtCl2(en)2](NO3)2 = Dichloridobis(ethane-1,2-diamine)platinum(IV) nitrate
Fe4[Fe(CN)6]3 = Iron(III) hexacyanoferrate(II)
[Co(NH3)5Cl]Cl2 = Pentaamminechloridocobalt(III) chloride
K3[Fe(C2O4)3] = Potassium trioxalatoferrate(III)
K2[PdCl4] = Potassium tetrachloridopalladate(II)
[Pt(NH3)2Cl(NH2CH3)]Cl = Diamminechlorido(methylamine)platinum(II)
chloride
10. [Co(NH3)6]2(SO4)3 = Hexaamminecobalt(III) sulphate
11. [Co(NH3)5(ONO)]2+ = Pentaamminenitrito-O-cobalt(III) ion
12. [Co(NH3)4Cl2]3[Cr(CN)6] =
Tetraamminedichloridocobalt(III)hexacyanochromate(III)
13. Na2[Fe(CN)5NO] = Sodium pentacyanonitrosoniumferrate(II)
14. K3[Co(CN)5NO] = Potassium pentacyanonitrosylcobaltate(II)
15. Na2[CrF4O] = Sodium tetrafluoridooxochromate(IV)
16. [Cr(H2O)4Cl2]NO3 = Tetraaquadichloridochromium(III) nitrate
17. (NH4)3[Cr(SCN)6] = Ammonium hexathiocyanato-S-chromate(III)
18. Na2[Cr(CH3COO)4(en)] = Sodium ethylenediaminetetraacetatochromate(II)
19. [Co(NH3)5(CO3)]Cl = Pentaamminecarbonatocobalt(III) chloride
20. [Pt(py)4][PtCl4] = Tetrapyridineplatinum(II)tetrachloridoplatinate(II)
21 LiAlH4 Lithium tetra hydrido Aluminate (III)
VBT:
Some limitations of VBT: i)
It does not explain the colour
exhibited by coordination
compounds. ii) It does not
distinguish between weak
and strong ligands. iii) It
does not give a quantitative
interpretation of the
thermodynamic or kinetic
stabilities of coordination
compounds. iv) It does not
give quantitative
interpretation of magnetic
data
CRYSTAL FIELD THEORY: The CFT considers the metal—ligand bond to be ionic
arising purely from electrostatic interactions between the metal ion and the ligand.
Ligands are treated as point charges in case of anions and as dipoles in case of
neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion are
degenerate. The degeneracy of the d orbitals is lifted when ligands approach the
metal atom/ion. This is due to ligand-electron metal-electron repulsions. It results
in splitting of the d orbitals into lower energy and higher energy orbitals. The pattern
of splitting depends upon the nature of the crystal field.
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If Äo < P, the fourth electron enters one of the eg orbitals giving the configuration
t2g3eg1 . Ligands for which Äo < P are known as weak field ligands and form high
spin complexes. (ii) If Äo > P, it becomes more energetically favourable for the
fourth electron to occupy a t2g orbital with configuration t2g 4eg0. Ligands which
produce this effect are known as strong field ligands and form low spin
complexes.
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The arrangement of ligands in order of their increasing CFSE values is known as
spectrochemical series. The ligands with small CFSE values are called weak
field ligands, whereas those with large value of CFSE are called strong field
ligands
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Synergic bonding interactions in a carbonyl complex: The bond between metal
and CO in metal carbonyls is strong due to the following reasons: i) The M—C
sigma bond is formed by the donation of lone pair of electrons on the carbonyl
carbon into a vacant d orbital of the metal. ii) The M—C pi bond is formed by
the donation of a pair of electrons from a filled d orbital of metal into the vacant
antibonding orbital of CO. Thus, the Metal to Ligand bonding creates a synergic
effect which strengthens the bond between CO and the metal
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Stability of a complex & factors affecting stability of a complex ion:. i)
Greater the charge and smaller the size of the central metal ion, more is its
charge density and greater will be the stability of the complex. ii) For different
metal ions with same charge, stability of the complexes increases as the size
decreases. iii) More the basicity of the ligand more is the stability of the
complex. iii) For anionic ligands the complexes are more stable if ligand has
greater charge and smaller size. iv) Chelating ligands form more stable
complexes.
ISOMERISM IN COORDINATION COMPOUNDS:
 Ionization isomerism : e.g. [Co(NH3)5Cl]2+SO42- and [Co(NH3)5SO4]+Cl Coordination
isomerism
:
e.g.
[Co(NH3)6][Cr(C2O4)3]
[Cr(NH3)6][Co(C2H4)3] .
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Linkage isomerism: e.g.[Co(NH3)5ONO]Cl2 and [Co(NH3)5NO2]Cl2
Nitritopentammine
Nitropentammine
Cobalt(III)chloride
Cobalt(III)chloride
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and
Solvate isomerism (hydrate isomerism) arises when water is involved as a
solvent. Solvate isomers differ by whether or not a solvent molecule is
directly bonded to the metal ion or merely present as free solvent molecules in
the crystal lattice
Geometrical
isomerism
or
cis-trans
isomerism:
 Optical isomerism :
VSA of One Mark
1. (each carry one mark)
2.
3. Define Spectrochemical series?
4. What is CFSE?
5. Draw the CFT for octahedral complexes.
6. Differentiate strong and weak field in coordination compounds?
7. Draw the structure of multi dentate ligand which is used for lead poisoning.
SA-1 of Two Marks
1. [NiCl4]2- is more likely to be tetrahedral while [Ni (CN) 4]2- is more likely to
be square planar. Explain
Ans:
2.
3.
4. Predict the Isomerism present in complexe like
SA-2 of Three Marks
1.
2. What is meant by CFSE on the basis of crystal field theory write the electronic
configuration of d4 in terms of t2g & eg in an octahedral field when i) 0> P
ii) ) 0<P
3. Name three factors that favours the formation of complexes by metal ion?
4. Describe the hybridization in following complexes.
(i) [Fe(CN)6]4– (ii) [FeF6]3–
HOTS
1.
2
3.
4. Predict the Isomerism present in complexe like