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UNIT-I
iii) Coordination Compounds
J.KAVITHA, M.Pharm.,
Lecturer,
Department of Pharmaceutical Analysis,
SRM College of Pharmacy
SRM University.
Contents
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2
3
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5
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25-7
Werner’s Theory of Coordination Compounds:
An Overview
Ligands
Nomenclature
Isomerism
Bonding in Complex Ions: Crystal Field Theory
Magnetic Properties of Coordination Compounds and
Crystal Field Theory
Color and the Colors of Complexes
Prentice-Hall ©
2002
General
Chemistry:
Sl
id
Werner’s Theory of Coordination Compounds: An Overview
• Compounds made up of simpler compounds are called coordination compounds.
• CoCl3 and NH3.
– CoCl3∙ (NH3)6 and CoCl3∙ (NH3)5.
– Differing reactivity with AgNO3.
Werner’s Theory
yTwo types of valence or bonding capacity.
◦ Primary valence.
x Based on the number of e‐ an atom loses in forming the ion.
◦ Secondary valence.
x Responsible for the bonding of other groups, called ligands, to the central metal atom.
[Co(NH3)6]Cl3 → [Co(NH3)6]3+ + 3 Cl[CoCl(NH3)5]Cl2 → [CoCl(NH3)5]3+ + 2 Cl-
Coordination Number
Example Charge on the metal ion:
2 Ligands
y Ligands are Lewis bases.
◦ Donate electron pairs to metals (which are Lewis acids).
y Monodentate ligands.
◦ Use one pair of electrons to form one point of attachment to the metal ion.
y Bidentate ligands.
◦ Use two pairs of electrons to form two points of attachment to the metal ion.
y Tridentate, tetradentate…..polydentate
Some Common Monodentate Ligands.
Some Common Polydentate Ligands (Chelating Agents)
3 Nomenclature
y In names and formulas of coordination compounds, cations come first, followed by anions.
y Anions as ligands are named by using the ending –o. ◦ Normally x – ide endings change to –o.
x – ite endings change to –ito.
x – ate endings change to –ato.
y Neutral molecules as ligands generally carried the unmodified name.
Nomenclature
• The number of ligands of a given type is given by a prefix.
• Mono, di, tri, tetra, penta, hexa…
– If the ligand name is a composite name itself
• Place it in brackets and precede it with a prefix:
– Bis, tris, tetrakis, pentakis... Nomenclature
• Name the ligands first, in alphabetical order, followed by the name of the metal centre.
– Prefixes are ignored in alphabetical order decisions.
• The oxidation state of the metal centre is given by a Roman numeral.
• If the complex is an anion the ending –ate is attached to the name of the metal.
Nomenclature
y When writing the formula
x the chemical symbol of the metal is written first,
x followed by the formulas of anions,
◦ in alphabetical order.
x and then formulas of neutral molecules,
◦ in alphabetical order.
4 Isomerism
y Isomers.
◦ Differ in their structure and properties.
y Structural isomers.
◦ Differ in basic structure.
y Stereoisomers.
◦ Same number and type of ligands with the same mode of attachement.
◦ Differ in the way the ligands occupy space around the metal ion.
Examples of Isomerism
Ionization Isomerism
[CrSO4(NH3)5]Cl
[CrCl(NH3)5]SO4
pentaaminsulfatochromium(III) chloride
pentaaminchlorochromium(III) sulfate
Coordination Isomerism
[Co(NH3)6][CrCN6]
[Cr(NH3)6][CoCN6]
hexaaminecobalt(III) hexacyanochromate(III)
hexaaminechromium(III) hexacyanocobaltate(III)
Geometric Isomerism
Optical Isomerism
5 Bonding in Complex Ions: Pauling’s Valence bond theory
y Consider bonding in a complex to be an electrostatic attraction between a positively charged nucleus and the electrons of the ligands.
◦ Electrons on metal atom repel electrons on ligands.
◦ Focus particularly on the d‐electrons on the metal ion.
DIMERCAPROL
Dimercaprol (or BAL or British Anti‐Lewisite) is a suphur analogue of glycerol and is
structural 2,3‐dimercaptopropanol. In the trihydric alcohol there are three hydroxyl groups.
But in dimercaprol, two of these hydroxyl groups are converted into –SH groups (thiol or
merdapto groups).
CH2OH‐CHOH‐CH2OH
Glycerol
Dimercaprol
CH2SH‐CHSH‐CH2OH
This was introduced in the second world war as an antidote by the British to the war gas
lewisite which was an arsenical. Since it contains the thiol (‐SH) groups, it combines with
the arsenic and so prevents or reverses the effect of arsenic on thiol‐ Conainging enzymes.
Dimercaprol forms poorly dissociating complexes also with mercury and cadmium ions.
Actullaydimercaprol is useful in the treatment of acute poisoning due to arsenic, mercury,
gold, antimony. Bismuth and thallium. It is given by injection. The injection is a sterile
solution of dimercaprol in a mixture of benzy I benzoate and arachis oil.
D-PENCICILLAMINE
D-penicillamine is chemically B,B-Dimethylysteine. It is derived from
the sulphur-containing aminoacid cysteine and is a monothiol. It is
prepared by the alkaline hydrolysis of benzylpenicillin.
It readily chelates with heavy metals such as mercury, copper and lead. It is used
in the treatment of wilson’s disease in which the copper content of the liver,
brain and other tissues is grossly increased it is also used in the treatment of
heavy metal poisoning such as acute lead poisoning and acute mercury
poisoning. D-Penicillamine is also used in the treatment of rheumatoid arthritis.
1,10 – PHENANTHROLINE
1,10 – Phenanthroline is a complexing agent which forms stable, highly coloured complexes
with a metal in two oxidation states with a suitable oxidation –reduction potential. So 1,10 –
phenanthroline-metal complex can be used as an oxidation –reduction indicator. 1,10orthophenanthroline has the following structure:-
It forms complexes with metals like iron,copper and zinc. Three molecules of 1,10phenanthroline dissolve easily in solutions of ferrous salts and combine with one ferrous ion to
give the 1,10-phecanthroline-ferrous complex known as ferroin which is intensely red in colour.
It has the following structure:It is oxidised to the pale blue ferric compound by any oxidizing agent such
as ceric ammonium sulphate or nitrate.Since the colour change is very
striking, ferroin is used as an indicator in redox titirations such as the assays
of ascorbic acid tablets, ferrous gluconate tablets, acetomenadione tablets
etc, at the end point ferroin is converted to the pale blue ferric complex.
[Fe(C12H8N2)3]2+
[Fe(C12H8N2)3]3+ + e-
EDTA
It has already been stated that aminopolycarboxylic acids are very goodcomplexing agents. They complex metals and fix them in uniomsable combination in 5 –or ‐6 membered rings which are quite stable, ethylene diaminetetraacetic acid is one such. Its structure is given below:‐
It forms water soluble complexes knowns as chelates with metals like Cu2+ and Mg2+
Disodium salt of ethylene diaminetetraacetic acid is a white, water- soluble dehydrate which
can be used as a primary standard, it is known as EDTA or disodium edetate or sodium
edetate. It is used in complexometric analysis as a titrant. It is non-specific in action and can
be used for the estimation of the alkaline earths and transition elements. It is simply known
as Na2H2Y and it ionizes in solution as H2Y2- which reacts quantitatively with may cations
such as Ca2+ and Mg2+, forming complexes. Other application of EDTA may be found in the
text.