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Coordination Compounds

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    COMPLEXOMETRIC TITRATION Course Contents: C OMPLEXOMETRIC TITRATION T ITRATION Complexation, Chelation. Werner’s co-ordination number, Titrants Stability of complexes, titration curves. Types of complexometric titrations. Methods of detecting end point, Standardization. ? Application of complexometric titrations. ? ? ? ? ? COMPLEXOMETRIC TITRATIONS COMPLEXOMETRIC TITRATIONS What is Complexometric Titration (CT)???? What What is a complex metal ion? A complex is a molecule or ion formed by the reaction of two or more ions or molecules capable of independent existence.

    What is COMPLEXATION????? Chemical reaction that takes place between the metal ion and a molecular or ionic entity (known as ligand). ? Also known as Chilometric titration, Chilometry, Chilatometric, EDTA titrations. ? It is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. ? The technique involves titrating metal ions with a complexing agent or chelating agent (Ligand) and is commonly referred to as complexometric titration. ? This method represents the analytical application of a complexation reaction. The endpoint is usually measured with an indicator ligand that forms a colored complex with the free metal ion. CT are useful for determination of mixture of different metal ions in solution. COMPLEXOMETRIC TITRATIONS 4 COMPLEXOMETRIC TITRATIONS 1 PAC-II COMPLEXOMETRIC TITRATION LIGANDS: Metal ion ( analyte ; cation) Metal ion indicators Chilon Complexing agent pM indicators ( Chelating agent L igand/Sequestering agent) Chelate Complex ion Metal co -ordination compound /Metal complex/Chelate compound ? Ligands or complexing or chelating agents are electron donating entity, which has the ability to ind to the metal ion and produce a complex ion. ? The molecules or ions which displace the solvent molecules are called Ligands. ? They are usually anions or polar molecules. COMPLEXOMETRIC TITRATIONS EXA E XA MP LES O F CO MP LEX IO N FO RMA T IO N COMPLEXOMETRIC TITRATIONS EXA E XA MP LES O F CO MP LEX IO N FO RMA TIO N i. [Fe(H2O)6]3+ Hexaaquairon(III) ion/ Iron(III) hexahydrated complex ion. ii. [AlF6]3Hexafluoroaluminate (III) ion iii. [Co(NH3)6]2+ Cobalt(II) hexaammino complex ion OR Hexaamminocobalt(II) ion iv. [Cu(NH3)4(H2O)2]2+ Tetra ammine copper(II) Hexaaqua cobalt (III) ion

    Tetra amminediaqua copper(II) 7 COMPLEXOMETRIC TITRATIONS 8 COMPLEXOMETRIC TITRATIONS 2 PAC-II COMPLEXOMETRIC TITRATION Bidentate and Multidentate Ligands: Classification of Ligands : Unidentate Ligands: Ligands that are bound to metal ion only at one place are called unidentate ligands (one toothed). Ex: water, ammonia, halide ions, cyanide ions 9 COMPLEXOMETRIC TITRATIONS Ligands that contain more than one donar group and capable of binding with metal ions as multidentate ligands or chelating agents. They include: ? bidentate ligands (2 donar atoms), ? tridentate ligands (3 donar atoms), quadridentate ligands (4 donar atoms) Eg: chlorophyll ? Hexadentate ligands (6 donar atoms) Eg: Ethylene diamine tetra acetic acid (EDTA 10 COMPLEXOMETRIC TITRATIONS E xamples Of M ultidentate L igands Multidentate Ligands SOME BIDENTATE LIGANDS Ethylenediammine (en) acetylacetonate ion (acac) Phenanthroline (phen) Oxalate ion (ox) 11 COMPLEXOMETRIC TITRATIONS 12 COMPLEXOMETRIC TITRATIONS 3 PAC-II COMPLEXOMETRIC TITRATION BONDING IN METAL COMPLEXES ? A central metal atom bonded to a group of molecules or ions is a metal complex. ?

    If the complex bears a charge, it is a omplex ion. ? Compounds containing complexes are known as coordination compounds. ? The bonds are either ordinary covalent bonds or co-ordinate bonds. ? Ordinary covalent bonds in which the metal and the ligand contribute one electron each. ? Co-ordinate covalent bonds in which both electrons are contributed by the ligand. Eg: Hexacyanoferrate ion consists of three ordinary covalent bonds and three co-ordinate bonds. The bonds in complex are identical hybrid bonds which are directed towards the apex of a regular octahedron. When neutral groups only are involved , the charge on the omplex is positive and is equal to the metal ion, e. g. [Cu(NH3)4]2+. ? ? ? COMPLEXOMETRIC TITRATIONS WERNER’S COORDINATION NUMBER (CN) Alfred Werner in 1893 suggested that metal ions exhibit two types of valances namely; COMPLEXOMETRIC TITRATIONS WERNER’S COORDINATION NUMBER (CN) ? The CN is specific for a given metal ion in a particular oxidation state and compound. ? CN range between 2 and 12, the most common are 4 (tetracoordinate) and 6 ( hexacoordinate). ? Some metals, such as chromium(III) and cobalt(III), have the same coordination number (6 in the case of these two metals).

    Primary valences refer to the oxidation number for the metal Secondary valences refer to the coordination number (CN) “the number of ligand atoms that are bonded directly to the central metal ion”. COMPLEXOMETRIC TITRATIONS COMPLEXOMETRIC TITRATIONS 4 PAC-II COMPLEXOMETRIC TITRATION CO O RD IN A TED C O MP LEXES A N D C O O RD IN A TIO N N UMBER Geometries CN ? There are two common geometries for metals with a coordination number of four: – Tetrahedral – Square planar ?Geometry for metal, when the coordination number is six, is octahedral. 2 4 Shape Example Linear [CuCl2]-, [Ag(NH3)2]+, AuCl2]- Square Planar [Ni(CN)4] 2-, [PdCl4]2- F F [Pt(NH3)4] 2+, [Cu(NH3)4] 2+ Br F F 4 6 Tetrahedral [Cu(CN)4] 3-, Zn(NH3)4]2+ [CdCl4] 2-, [MnCl4] 2- Octahedral F F F S F F COMPLEXOMETRIC TITRATIONS [Cu(H2O)6] 3+, [V(CN)6] 4-, [Cu(NH3)4Cl2] +, [Co(en)3] 3+ F COMPLEXOMETRIC TITRATIONS 18 BIOLOGICAL COORDINATING COMPLEXES Many naturally occurring substances exits as coordination compounds : 1. Haemoglobin ( a complex of Fe2+) 2. Chlorophyll ( a complex of Mg2+) 3. Vitamin B12 (a complex of Co2+) Hemoglobin Chlorophyll Vitamin B12 COMPLEXOMETRIC TITRATIONS 20 COMPLEXOMETRIC TITRATIONS 5

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