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

Outline Concept of complex compounds and complexing process. Types of complexes. Stability of complexes and influence of different factors on it. Influence of complexing on precipitate solubility and oxidation-reduction potential of system. Usage of complexing in analytical chemistry.

1. Concept of complex compounds and complexing process 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-] Malachite green

2. Complexes without the coordination centre Hydroquinone Quinhydrone Quinone

3. Coordination complex compounds

Coordination complex compounds: 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 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. Diglycinatocopper (ІІ) Diethylenediaminocopper (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 cumulative (common) MeLn  MeLn-1+ L MeLn  Me + nL MeLn-1 MeLn-2+ L МeLn-1  Me + (n-1)L ……………………. …………………….. MeL2  MeL + L MeL2  Me + 2L MeL Me + L 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 MeLn↔Me + nL 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 redox-pair can increase or decrease (depending on the nature of comlexes, which will form with oxidizing and reduction redox-pair forms)

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).

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