addition compound , double salt, types of ligands , ionic sphere,examples, additional questions , naming of co ordination compound , naming og central meta atom, donor atom,special names ,rules of naming ,
1. Introduction
(a) The concept of coordination compounds arises from the complex formation tendency
of transition elements.
(b) These compounds play a vital role in our lives, as chlorophyll of plants and haemoglobin
of animal blood are also coordination compounds of Mg and Fe respectively
(c) One of the earliest known coordination compound is prussian blue which was accidently
prepared by a Berlin color maker, Diebach in 1704.
(d) The field of such compound is expanding due to their utilization in metallurgy, analytical
chemistry and polymerisation.
2. Molecular or addition compounds
(a) When solutions containing two or more simple stable compounds in molecular
proportions are allowed to evaporate, crystals of new substances are obtained. These
substances are termed as molecular or addition compounds
(b) Some common examples are as follows:
CuSO4 + 4NH3 CuSO4 . 4NH3
AgCN + KCN KCN . AgCN
Simple stable Addition or molecular
compound compounds
4 KCN + Fe(CN)2 Fe(CN2). 4KCN
K2SO4 + Al2 (SO4)3 + 24H2O K2SO4 . Al2(SO4)3 . 24 H2O
(c) The molecular or addition compounds are of two types double salts lattice compound
and coordination or complex compound
2.1 Double salts or lattice compounds
(a) The addition compounds which are stable in solid state only but are broken down into
individual constituents when dissolved in water are called soluble salts or lattice
compounds.
(b) For example when carnallite (KCl.MgCl2.6H2O) is dissolved in water it exhibits the
properties of KCl and MgCl2
2.2 Coordination or complex compounds
(a) The addition compounds in which some of the constituents ions or molecules lose their
identity and when dissolved in water they do not break up completely into individual
ions are called coordination compounds.
(b) The properties of their solutions are different than those of their constituents.
(c) In such compounds there is complex ion which is a central metal ion with lewis bases
attached to it through coordinate covalent bonds.
(d) On the basis of stability of complex ion, complex ions are further divided as follows:
(i) Perfect complexes : Those in which complex ion is fairly stable and is either not
dissociated or feebly dissociated in solution state, e.g.
K4[Fe(CN)6] 4K+ + [Fe(CN)6]4–
Fe2+ + 6CN¯ (Feebly dissociated)
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The ferrocyanide ion [Fe(CN)6]4– is so insignificantly dissociated that it can be considered as
practically undissociated and does not give the test of Fe2+ or CN¯ ions
(ii) Imperfect complexes: Those in which complex ion is less stable and is reversibly dissociated
to give enough simple ions and thus imparts their tests, e.g
K2[Cd(CN)4] 2K+ + [Cd(CN)4]2–
Cd2+ + 4CN¯ (Appreciably dissociated)
Note : An imperfect complex may be too unstable to exist and may be completely dissociated
in solution, it then becomes a double salt
3. Various terms used in coordination compounds
To understand concepts related to coordination compounds and their nature it is essential to
discuss some important terms. These are as follows:
3.1 Central ion : (Centre of coordination)
(a) The cation to which one or more neutral molecules or anions are attached is called as
centre of coordination
(b) Since the central ion acts as a acceptor and thus has to accommodate electron pairs
donated by the donor atom of the ligands, it must have empty orbitals
(c) This explains why the transition metals having empty d-orbitals form coordination
compounds very readily.
(d) In the complexes [Ni(NH3)6]2+ and [Fe(CN)6]3–, Ni2+ and Fe3+ respectively are the
central ions
3.2 Ligands
(a) The neutral molecules, anions or cations which are directly linked with the central metal
atom or ion in a complex ion are called ligands.
(b) Ligand donate a pair of electrons to the central atom
(c) Ligands are attached to the central metal ion or atom through coordinate bonds or
dative linkage
(d) Ligands are normally polar molecules like NH3 ; H2O or anions such as Cl¯, OH¯,
CN¯ etc. which contain at least one unshared pair of valency electrons.
(e) With few exceptions, free ligands have one pair of electrons that is not engaged in
bonding for example.
:CN:¯, Cl ¯
3.2.1 Types of Ligands on the basis of number of donor atoms present in them :
(a) Mono- or unidentate ligands: They have one donor atom, i.e. they supply only one
examples : F¯, Cl¯, Br¯, H2O , NH3, CN¯, NO2¯, NO¯ CO
(b) Bidentate ligands: Ligands which have two donor atoms and have the ability to link
with central metal ion at two positions are called bidentate ligands. Some examples are:
CH2 N
(1) (2) oxalate(ox) (3) 1,10-phenanthroline (o-phen)
CH2 N
Ethylenediamine (en)
(4) Glycinato (Gly) (5) 2,2'-Dipyridyl (Dipy) (6) Carbonato
(c) Tridentate ligands : The ligands having three donor atoms are called tridentate ligands.
Examples are:
N N
(1) (2) Diethylene triamine (dien)
N
2,2',2"-Terpyridine (terpy)
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(d) Tetradentate ligands: These ligands possess four donor atoms. Example are:
CH2COO¯
CH2COO¯
(1) N (2) Triethylene tetramine (Trien)
CH2COO¯
(Nitriloacetato)
(e) Pentadentate ligands : They have five donor atoms. For example, ethylenediamine
triacetate ion.
Ethylenediamine triacetato ion
(f) Hexadentate ligands : They are six donor atoms. The most important example is
ethylenediamine tetraacetateion.
Ethylenediamine tetraacetato ion (EDTA)4–
(g) Chelating ligands:
(a) Polydentate ligands whose structures permit the attachment of two or more donor sites
to the same metal ion simultaneously, thus closing one or more rings are called chelating
ligands and the compounds formed are known as chelate compounds.
(b) A chelate may be defined as a ring structure formed by the combination of a polydentate
ligand having two or more donor atoms with a metal ion forming part of the ring.
(c) The process of formation of chelates is called chelation.
(d) Chelate complexes are more stable than ordinary complexes in which the ligands is a
monodentate
(e) This increased stability of the compound due to chelation is called the chelate effect
(f) In the complex ion given below, 5 membered rings are formed. So all these are called
chelate complexes
2+
CH2—H2N NH2—CH2
Cu
CH2—H2N NH2—CH2
(g) Generally the chelate complexes with 5 or 6 membered rings are more stable.
(h) Flexidentate ligands: Polydentate ligands have flexidentate character. It is not
necessary that all the donor atoms present in the polydentate donor atoms present in
the polydentate ligands should form coordinate bonds with central metal atom (on ion).
For example: EDTA which is hexadentate ligand can functions as pentadentate (on
tetradentate ligand with certain metal ions. Finaly sulphate ion can also act as
monodentate ligand.
(i) Out of these, 5 membered rings are very stable when they involve saturated ligands
(j) On the other hand 6-membered ring structures acquire maximum stability when they
involved unsaturated ligands containing conjugate double bond. This is due to the
resonance effects involving metal d-orbitals and ligand p–orbital electrons.
(k) Ambidentate ligands: There are certain ligands which have two or more donor atoms
but during formation of complexes only one donor atoms is attached to metal ion. Such
ligands are called ambidentate ligands such as CN¯, NC¯.
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