Л Е К Ц ІЯ 4 Complexation equilibrium L e c t u r e 3L e c t u r e 3L e c t u r e 3L e c t u r e 3 Associate prof. L.V. Vronska Associate prof. L.V. Vronska.

Л Е К Ц ІЯ 4Л Е К Ц ІЯ 4

Complexation equilibriumComplexation equilibrium

LL ee cc tt uu rr ee 3 3

Associate profAssociate prof . L.V. Vronska. L.V. VronskaAssociate profAssociate prof . M.M. Mykhalkiv

OutlineOutline

1.1. Concept of complex compounds and Concept of complex compounds and

complexing process. Types of complexes.complexing process. Types of complexes.

2.2. Stability of complexes and influence of Stability of complexes and influence of

different factors on it.different factors on it.

3.3. Influence of complexing on precipitate Influence of complexing on precipitate

solubility and oxidation-reduction potential of solubility and oxidation-reduction potential of

system.system.

4.4. Usage of complexing in analytical chemistry.Usage of complexing in analytical chemistry.

1. 1. Concept of complex compounds and Concept of complex compounds and complexing process. Types of complexes.complexing process. Types of complexes.

ComplexesComplexes are multiple objects, which are are multiple objects, which are formed of more simple objects (ions, formed of more simple objects (ions, molecules), capable to independent existence in molecules), capable to independent existence in solutions.solutions.

ComplexingComplexing – it is– it is aa process of complex process of complex compounds formation from more simple compounds formation from more simple objects.objects.

The term The term complexcomplex in chemistry is usually used in chemistry is usually used to describe molecules or ensembles formed by to describe molecules or ensembles formed by the combination of the combination of ligandsligands and metal ions. and metal ions.

The molecules or ions that surround the central The molecules or ions that surround the central metal ion in a coordination compound are called metal ion in a coordination compound are called ligandsligands, , and the atoms that are attached directly and the atoms that are attached directly to the metal are called ligand to the metal are called ligand donor atomsdonor atoms..

The number of ligand donor atoms that surround The number of ligand donor atoms that surround a central metal ion in a complex is called the a central metal ion in a complex is called the coordination numbercoordination number of the metalof the metal

Originally, a complex implied a reversible Originally, a complex implied a reversible association of molecules, atoms, or ions through association of molecules, atoms, or ions through weak chemical bonds.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 CompoundsNaming Coordination Compounds

Names of Some Common Metallate Names of Some Common Metallate AnionsAnions

Names of Some Common LigandsNames of Some Common Ligands

Examples of Complexes withExamples of Complexes withVarious Coordination NumbersVarious 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 monodentate or or polydentatepolydentate,, 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 monodentatemonodentate ligands (literally, “onetoothed” ligands).

Those that bond through electron pairs on more than one donor atom are termed polydentatepolydentate ligands (“many-toothed” ligands).

For example, ethylenediamine (NH2CH2CH2NH2 abbreviated en) is a bidentatebidentate ligand because it bonds to a metal using an electron pair on each of its two nitrogen atoms.

The hexadentatehexadentate 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.Ligand donor atoms are in color.

Types of complexTypes of complex::

1. 1. Ionic associates (ionic pairs)Ionic associates (ionic pairs) in solutions are in solutions are formed as a result only electrostatic interaction formed as a result only electrostatic interaction between opposite charged ions, for examplebetween opposite charged ions, for example

KtKt++ ++ AnAn--[Kt[Kt++, An, An--]] (CH3)2N N(CH3)2

C

(CH3)2N

C

N(CH3)2

[SbCl6]-+

+

[SbCl6]-

+

Malachite green

2. 2. Complexes without the coordination centreComplexes without the coordination centre

Hydroquinone Hydroquinone Quinhydrone Quinhydrone

QuinoneQuinone

3. 3. Coordination complex compoundsCoordination complex compounds

Coordination complex Coordination complex compounds:compounds:

1.1. One-nuclear complexesOne-nuclear complexes One-ligandly: One-ligandly: metallamine metallamine [Cu(NH[Cu(NH33))44]SO]SO44

aquacomlexes aquacomlexes [Co(H[Co(H22O)O)66]Cl]Cl22

acidocomplexes acidocomplexes KK22[PtCl[PtCl44]; ];

HH22[SiF[SiF66];];

Combination-ligandly: Combination-ligandly: [Pt(NH[Pt(NH33)Cl)Cl22];];

[Pt(NH[Pt(NH33)Cl)Cl33].].

2. Poly-nuclear complexes2. Poly-nuclear complexes bridging complex bridging complex [Cr(NH3)5-OH-(NH3)5Cr]Cl5[Cr(NH3)5-OH-(NH3)5Cr]Cl5 cluster complex cluster complex

isopoly acids isopoly acids НН44РР22ОО77, Н, Н22ВВ44ОО77

heteropoly acids heteropoly acids HH33POPO44·12MoО·12MoО33·nН·nН22OO

HH33POPO44·12WО·12WО33·nН·nН22O O

HH44SiОSiО44·12MoО·12MoО33·nН·nН22OO

HH44SiОSiО44·12WО·12WО33·nН·nН22OO

Re Re

Br Br

Br

Br

Br BrBr

Br

2-

A complex such as [Co(en)3]3+ or Co(EDTA)]- that contains one or more chelate rings is known as a metal chelatechelate.

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.chelate ring.

[Co(en)3]3+ Co(EDTA)]-

Inner-complexInner-complex compounds contain ionic and compounds contain ionic and donor-acceptor bonds.donor-acceptor bonds.

Scheme of copper Scheme of copper chelationchelation [[CuCu((NHNH33))44]]2+2+

Octahedral structure of theOctahedral structure of the[Co(NH[Co(NH33))66]]3+3+

Idiosyncrasy of chelate – it is presence Idiosyncrasy of chelate – it is presence of cycles.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)3

3-. (e) After adding HgCl2 to the original solution, the red color again vanishes.

Necessary parts of ligands for chelate Necessary parts of ligands for chelate formationformation

1. 1. Functional-analytical groupsFunctional-analytical groups (FAG) (FAG) - - areare specific groups which provide occurrence of specific groups which provide occurrence of donor-acceptor bond.donor-acceptor bond.

-ОН, -SH, =NH, -COOH, -SO-ОН, -SH, =NH, -COOH, -SO33H, -AsОH, -AsО33HH22, ,

C=Ö: і т.д.C=Ö: і т.д.

2. 2. Analytical-active groupsAnalytical-active groups (ААG) (ААG) – are the groups – are the groups of atoms which change of atoms which change analyticalanalytical properties of properties of reaction products (solubility, intensity of reaction products (solubility, intensity of colouring). colouring).

AuxochromeAuxochrome - this is a group of atoms attached to a - this is a group of atoms attached to a chromophore which modifies the ability of that chromophore which modifies the ability of that chromophore to absorb light. chromophore to absorb light.

An auxochrome is a functional group of atoms with An auxochrome is a functional group of atoms with nonbonded electrons which, when attached to a nonbonded electrons which, when attached to a chromophore, alters both the wavelength and chromophore, alters both the wavelength and intensity of absorption. intensity of absorption.

If these groups are in direct conjugation with the If these groups are in direct conjugation with the pi-systempi-system of the chromophore, they may increase of the chromophore, they may increase the wavelength at which the light is absorbed and the wavelength at which the light is absorbed and as a result intensify the absorption as a result intensify the absorption (-Cl, -Br, -J, -(-Cl, -Br, -J, -CC66HH55)). .

A feature of these auxochromes is the presence of A feature of these auxochromes is the presence of at least one lone pair of electrons which can be at least one lone pair of electrons which can be viewed as extending the conjugated system by viewed as extending the conjugated system by resonance. Also that groups which improve resonance. Also that groups which improve solubility of complexes solubility of complexes (-SO(-SO33H,-COOH).H,-COOH).

Process of complexingProcess of complexing stepwise fashionstepwise fashion cumulative cumulative

(common)(common)

Me + L ↔ MeL Me + L ↔ MeL Me + L ↔ MeL Me + L ↔ MeL

MeL + L ↔ MeLMeL + L ↔ MeL22 Me + 2L ↔ MeL Me + 2L ↔ MeL22

MeLMeL22 + L ↔ MeL + L ↔ MeL33 Me + 3L ↔ MeL Me + 3L ↔ MeL33

·································· ·································· ·································· ··································

MeLMeLn-1n-1+ L ↔ MeL+ L ↔ MeLnn Me + n L ↔MeL Me + n L ↔MeLnnThe formation of a metal–ligand complex is described by a formation constant, Kf.

Process of complex Process of complex dissociatedissociatestepwise fashionstepwise fashion cumulative (common)cumulative (common)

MeLMeLnn MeL MeLn-1n-1+ L + L MeL MeLnn Me + nL Me + nL

MeLMeLn-1n-1 MeL MeLn-2n-2+ L МeL+ L МeLn-1 n-1 Me + (n-1)L Me + (n-1)L

………………………………………….. ……………………..……………………..

MeMeLL22 MeL + L MeL + L Me MeLL22 Me + 2L Me + 2L

MeMeLL Me + L Me + L MeMeLL Me Me +L+L The reverse of reaction complexing is called a

dissociation reaction and is characterized by a dissociation constant, Kd

Stepwise formation constantsThe 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 ↔MeLMe + nL ↔MeLnn MeL MeLnn↔Me + nL ↔Me + nL

β (Kf) - formation constant (or stability constant)

! So,! So, Kd, which is the reciprocal of Kf.

]MeL[

]L[]Me[K

n

n'd

n

n

n ]L[]Me[]MeL[

'd

n K

1

2. 2. Stability of complexes and influence Stability of complexes and influence of different factors on it.of different factors on it.

Kinetic stability:Kinetic stability: Labile complexesLabile complexes Inert complexesInert complexes

Thermodynamic stability:Thermodynamic stability:

formation constant ((dissociation constant))

FactorsFactors which influence which influence stability of complex stability of complex

connections:connections:

The ion natureThe ion nature of metal and ligand; of metal and ligand;

The chargeThe charge of an metal ion; of an metal ion;

Ionic radiusIonic radius of the metal-complexing agent; of the metal-complexing agent;

The natureThe nature of medium. of medium.

Influence of different factors on Influence of different factors on complexing in solution.complexing in solution.

1. Ionic strength of solution1. Ionic strength of solution

2. рН 2. рН

3. concentration of ligand3. concentration of ligand

4. temperature4. temperature

5. stranger ions, which form slightly soluble 5. stranger ions, which form slightly soluble compound with metal-complexing agent or compound with metal-complexing agent or ligand.ligand.

3. 3. Influence of complexing on precipitate Influence of complexing on precipitate solubility and oxidation-reduction solubility and oxidation-reduction

potential of system.potential of system.

the solubility of precipitate increasesthe solubility of precipitate increases oxidizing and reducing properties of redox-oxidizing and reducing properties of redox-

pair pair can increase or decreasecan increase or decrease (depending (depending on the nature of comlexes, which will form on the nature of comlexes, which will form with oxidizing and reduction redox-pair with oxidizing and reduction redox-pair forms)forms)

4. 4. Usage of complexing in analytical Usage of complexing in analytical chemistry.chemistry.

masking of іоnsmasking of іоns determination of cations and anionsdetermination of cations and anions separationseparation concentrating and determination of ionsconcentrating and determination of ions precipitation of cations and anions from the solutionsprecipitation of cations and anions from the solutions dissolution of precipitatedissolution of precipitate definition identity of drugs on functional groupsdefinition identity of drugs on functional groups change red-ox potentialchange red-ox potential determination of ions by fluorescence analysisdetermination of ions by fluorescence analysis for fixing of equivalence point in titrimetric analysisfor fixing of equivalence point in titrimetric analysis

Silver chloride is insoluble in water (left) but dissolves on addition of an excess of aqueous ammonia (right).

The qualitative analysisThe 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 analysisThe 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)4

2+ ion (right).

The qualitative analysisThe qualitative analysis

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