Download 03 Complexation equilibrium

Lecture3
Л Е К Ц ІЯ 4
Complexation equilibrium
Associate prof . L.V. Vronska
Associate prof . M.M. Mykhalkiv
Outline
1. Concept of complex compounds and
complexing process. Types of complexes.
2. Stability of complexes and influence of
different factors on it.
3. Influence of complexing on precipitate
solubility and oxidation-reduction potential of
system.
4. Usage of complexing in analytical chemistry.
1. Concept of complex compounds and
complexing process. Types of complexes.
Complexes are multiple objects, which are
formed of more simple objects (ions,
molecules), capable to independent existence in
solutions.
Complexing – it is a process of complex
compounds formation from more simple
objects.
 The term complex in chemistry is usually used
to describe molecules or ensembles formed by
the combination of ligands and metal ions.
 The molecules or ions that surround the central
metal ion in a coordination compound are called
ligands, and the atoms that are attached directly
to the metal are called ligand donor atoms.
 The number of ligand donor atoms that surround
a central metal ion in a complex is called the
coordination number of the metal
 Originally, a complex implied a reversible
association of molecules, atoms, or ions through
weak chemical bonds.
Aqueous solutions that contain [Ni(H2O)6]2+, [Ni(NH3)6]2+ and [Ni(en)3]2+
(from left to right). The two solutions on the right were prepared by
adding ammonia and ethylenediamine, respectively, to aqueous
nickel(II) nitrate.
Naming Coordination Compounds
Names of Some Common Metallate
Anions
Names of Some Common Ligands
Examples of Complexes with
Various Coordination Numbers
 Ligands have at least one lone pair of electrons
that can be used to form a coordinate covalent
bond to a metal ion.
 They can be classified as monodentate or
polydentate, depending on the number of ligand
donor atoms that bond to the metal.
Ligands such as H2O, NH3 or Cl- that bond using the
electron pair of a single donor atom are called
monodentate ligands (literally, “onetoothed”
ligands).
 Those that bond through electron pairs on more than
one donor atom are termed polydentate ligands
(“many-toothed” ligands).
For example, ethylenediamine (NH2CH2CH2NH2
abbreviated en) is a bidentate ligand because it
bonds to a metal using an electron pair on each of its
two nitrogen atoms.
 The hexadentate ligand ethylenediaminetetraacetate
ion (EDTA4-) bonds to a metal ion through electron
pairs on six donor atoms (two N atoms and four O
atoms).
Structures of some common ligands
Ligand donor atoms are in color.
Types of complex:
1. Ionic associates (ionic pairs) in solutions are
formed as a result only electrostatic interaction
between opposite charged ions, for example
Kt+ + An-[Kt+, An-]
+
(CH3)2N
N(CH3)2
C
-
[SbCl6] +
Malachite green
+
(CH3)2N
N(CH3)2
C
-
[SbCl6]
 2. Complexes without the coordination
centre
Hydroquinone
Quinhydrone
Quinone
 3. Coordination complex compounds
Coordination complex
compounds:
1. One-nuclear complexes

One-ligandly: metallamine [Cu(NH3)4]SO4
aquacomlexes [Co(H2O)6]Cl2
acidocomplexes K2[PtCl4]; H2[SiF6];
 Combination-ligandly: [Pt(NH3)Cl2];
[Pt(NH3)Cl3].
2. Poly-nuclear complexes
 bridging complex [Cr(NH3)5-OH-(NH3)5Cr]Cl5
 cluster complex
Br
Br
Br
Re
Br
2-
Br
Re
Br
Br
Br
 isopoly acids
Н4Р2О7, Н2В4О7
 heteropoly acids
H3PO4·12MoО3·nН2O
H3PO4·12WО3·nН2O
H4SiО4·12MoО3·nН2O
H4SiО4·12WО3·nН2O
 A complex such as [Co(en)3]3+ or Co(EDTA)]- that
contains one or more chelate rings is known as a metal
chelate.
The resulting five-membered ring consisting of the Co(III)
ion, two N atoms, and two C atoms of the ligand is called
a chelate ring.
[Co(en)3]3+
Co(EDTA)]-
 Inner-complex compounds contain ionic and
donor-acceptor bonds.
Scheme of copper chelation [Cu(NH3)4]2+
Octahedral structure of the
[Co(NH3)6]3+
Idiosyncrasy of chelate – it is presence
of cycles.
Diethylenediaminocopper (ІІ)
Diglycinatocopper (ІІ)
active site of chlorophyll
active site of hemoglobin
hemoglobin
Structure of molecule of cyancobalamin
(vitamin В12)
Mechanism of action Tetacinum-calcium
Ions Hg2+ and Cd2+ displace ions Ca2+ from
Tetacinum
Color changes produced by adding various reagents to an
equilibrium mixture of Fe3+ (pale yellow), SCN- (colorless),
and FeNCS2+ (red): (a) The original solution. (b) After
adding to FeCl3 the original solution, the red color is darker
because of an increase in [FeNCS2+]. (c) After adding
KSCN to the original solution, the red color again deepens.
(d) After adding H2C2O4 to the original solution, the red
color disappears because of a decrease in [FeNCS2+] the
yellow color is due to Fe(C2O4)33-. (e) After adding HgCl2
to the original solution, the red color again vanishes.
Necessary parts of ligands for chelate
formation
1. Functional-analytical groups (FAG) - are
specific groups which provide occurrence of
donor-acceptor bond.
-ОН, -SH, =NH, -COOH, -SO3H, -AsО3H2,
C=Ö: і т.д.
2. Analytical-active groups (ААG) – are the groups
of atoms which change analytical properties of
reaction products (solubility, intensity of
colouring).
Auxochrome - this is a group of atoms attached to a
chromophore which modifies the ability of that
chromophore to absorb light.
An auxochrome is a functional group of atoms with
nonbonded electrons which, when attached to a
chromophore, alters both the wavelength and
intensity of absorption.
If these groups are in direct conjugation with the
pi-system of the chromophore, they may increase
the wavelength at which the light is absorbed and
as a result intensify the absorption (-Cl, -Br, -J, C6H5).
A feature of these auxochromes is the presence of at
least one lone pair of electrons which can be
viewed as extending the conjugated system by
resonance. Also that groups which improve
solubility of complexes (-SO3H,-COOH).
Process of complexing
stepwise fashion
cumulative (common)
Me + L ↔ MeL
Me + L ↔ MeL
MeL + L ↔ MeL2
Me + 2L ↔ MeL2
MeL2 + L ↔ MeL3
Me + 3L ↔ MeL3
··································
··································
MeLn-1+ L ↔ MeLn
Me + n L ↔MeLn
The formation of a metal–ligand complex is
described by a formation constant, Kf.
Process of complex dissociate
stepwise fashion
MeLn  MeLn-1+ L
MeLn-1 MeLn-2+ L
…………………….
MeL2  MeL + L
MeL Me + L
cumulative (common)
MeLn  Me + nL
МeLn-1  Me + (n-1)L
……………………..
MeL2  Me + 2L
MeL  Me +L
The reverse of reaction complexing is called a
dissociation reaction and is characterized by a
dissociation constant, Kd
 Stepwise formation constants
The formation constant for a metal–ligand
complex in which only one ligand is added
to the metal ion or to a metal–ligand
complex (Ki)
 Cumulative formation constant
The formation constant for a metal–ligand
complex in which two or more ligands are
simultaneously added to a metal ion or to a
metal–ligand complex (βi).
For example, the reaction between Cd2+ and
NH3 involves four successive reactions
So
Relationship between Kf() and Kd
Me + nL ↔MeLn
[MeL ]
 
[Me]  [L]
MeLn↔Me + nL
'
d
n
n
[ Me ]  [ L ]
K 
[ MeLn ]
n
n
1
n  '
Kd
 β (Kf) - formation constant (or stability constant)
! So, Kd, which is the reciprocal of Kf.
2. Stability of complexes and influence
of different factors on it.
Kinetic stability:
 Labile complexes
 Inert complexes
Thermodynamic stability:
 formation constant (dissociation constant)
Factors which influence stability of complex
connections:
 The ion nature of metal and ligand;
 The charge of an metal ion;
 Ionic radius of the metal-complexing agent;
 The nature of medium.
Influence of different factors on
complexing in solution.
1. Ionic strength of solution
2. рН
3. concentration of ligand
4. temperature
5. stranger ions, which form slightly soluble
compound with metal-complexing agent or
ligand.
3. Influence of complexing on precipitate
solubility and oxidation-reduction
potential of system.
 the solubility of precipitate increases
 oxidizing and reducing properties of redoxpair can increase or decrease (depending
on the nature of comlexes, which will form
with oxidizing and reduction redox-pair
forms)
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4. Usage of complexing in analytical
chemistry.
masking of іоns
determination of cations and anions
separation
concentrating and determination of ions
precipitation of cations and anions from the solutions
dissolution of precipitate
definition identity of drugs on functional groups
change red-ox potential
determination of ions by fluorescence analysis
for fixing of equivalence point in titrimetric analysis
The qualitative analysis
Silver chloride is insoluble in water (left) but
dissolves on addition of an excess of
aqueous ammonia (right).
The qualitative analysis
Aluminum hydroxide, a gelatinous white precipitate, forms
on addition of aqueous NaOH to Al3+ (aq). (b) The
precipitate dissolves on addition of excess aqueous NaOH,
yielding the colorless soluble complex ion [Al(OH)4]+. (The
precipitate also dissolves in aqueous HCl, yielding the
colorless Al3+ ion.)
The qualitative analysis
When an aqueous solution of CuSO4 (left) is treated with
aqueous ammonia, a blue precipitate of Cu(OH)2 forms
(center). On the addition of excess ammonia, the precipitate
dissolves, yielding the deep blue Cu(NH3)42+ ion (right).
Thanks for your attention!