Title SP-4_Ch-20_Coordination Compounds.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Introduction Compounds formed due to combination of two or more simple stable salts, which retain their identity in solid as well as in dissolved state, are called coordination compounds. These are backbone of modern inorganic and bio-inorganic chemistry and chemical industry. Chlorophyll, haemoglobin and vitamin B12 are co-ordination compounds of magnesium, iron and cobalt respectively. In this chapter, you will be able to learn about the terms : Coordination entity, central atom/ion, ligand, coordination number, types of complexes etc. You will also learn about IUPAC nomenclature of coordination compounds, theories of bonding in terms of Werner’s theory, valence bond theory and crystal field theory. A lot of discussion will be done about types of isomerism, stability of complexes and organometallic compounds. You are advised to do practice on the structure of complexes and their methods of formation. DEFINITIONS OF SOME IMPORTANT TERMS PERTAINING TO COORDINATION COMPOUNDS 1. Addition Compound Compounds made up of two or more stable compounds by crystallization in a fixed stoichiometric ratio are called addition compound. For example KCl + MgCl2 + 6H2O  KCl  MgCl2  6H2O (carnallite) CuSO4 + 4NH3 + H2O  CuSO4  4NH3  H2O (tetrammine copper(II) sulphate monohydrate) Addition compounds are of two types : (i) Those which lose their identity in solution (double salt) FeSO4  (NH4) 2SO4  6H2O  Fe2+ + 2NH4 + + 2SO4 2– + 6H2O (Mohr’s salt) Chapter 20 Coordination Compounds Introduction Definitions of Some Important Terms Pertaining to Coordination Compounds Classification of Ligands Werner’s Theory of Coordination Compounds Nomenclature of Coordination Compounds Isomerism in Coordination Compounds Bonding in Coordination Compounds Bonding in Metal Carbonyls Stability of Coordination Compounds Coordination Compounds in Daily Life
2 Coordination Compounds NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (ii) Those which retain their identity in solution (coordination or complex compounds) Fe(CN) + 4KCN 2 K [Fe(CN) ] 4 6 4K + [Fe(CN) ] + 4– 6 (Potassium ferrocyanide) 2. Coordination Entity or coordination sphere : It constitutes a central metal atom or ion bonded to a fixed number of ions or molecules with coordinate bonds. For example, in coordination sphere (entity) of [CoCl3 (NH3 ) 3 ], cobalt ion (Co3+) is surrounded by three ammo- nia molecules and three chloride ions. 3. Central Atom/Ion In coordination entity, the atom/ion to which a fixed number of ions/groups are attached in a definite geometrical arrangement around it, is called central atom/ion. For example, in K2[PtCl6], Pt+4 is central metal ion. Central metal atom or ion accepts lone pair of electrons from the ligands hence it acts as Lewis acid. 4. Ligands The donor atoms, molecules or anions which donate a pair of electrons to the metal atom/ion are called ligands. Hence ligands are Lewis bases. For example, in [Ni(NH3) 6]Cl2, NH3 is ligand (Lewis base). 5. Coordination Number It may be defined as number of coordinate bonds formed with central atom/ion by the ligands. For example in coordination entity [Ag(CN)2] –,[Cu(NH3) 4] 2+ and [Cr(H2O)6] 3+, the coordination number of Ag, Cu and Cr are 2, 4 and 6 respectively. Similarly in [Fe(C2O4) 3] 3– and [Co(en)3] 3+, the coordination number of both Fe and Co is 6 because C2O4 2– (oxalate) and en (ethylene-1, 2-diamine) forms two coordinate bonds each with central metal atom/ion. Note : Coordination number of central atom/ion is determined only by number of sigma bonds formed by ligands with central metal atom/ion. Pi bonds, if formed, are not counted for this purpose. The ionizable groups are written outside the square bracket (in outer sphere) and are called counter ions. For example, in K4[Fe(CN)6] and [Cu(NH3)4]SO4, counter ion is K+ and SO4 2– respectively. 6. Coordination Polyhedron The spatial arrangement of ligands around central metal atom/ion is called coordination polyhedron. The most common coordination polyhedra are M L L L L L L M L L L L L M L L L L Octahedral Square pyramidal Trigonal bipyramidal Tetrahedral Square planar M L L L L M L L L L L Fig.: Shapes of different coordination polyhedra. M represents the central atom/ion and L, a unidentate ligand 7. Oxidation Number Oxidation number of the central metal atom/ion in a complex is the charge present on it if all the ligands are removed along with the electron pairs that are shared with central atom. It is represented by Roman
NEET Coordination Compounds 3 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 numerals in parenthesis after the name of central atom. For example, oxidation number of Co, Fe and Ni in [Co(NH3) 6] 3+, [Fe(CN)6] 4– and [Ni(CO)4] is +3, +2 and 0, and written as Co(III), Fe(II) and Ni(0) respectively. 8. Homoleptic and Heteroleptic Complexes Complexes which have only one type of ligands are homoleptic e.g., [Co(NH3) 6] 3+ and have more than one type of ligands are heteroleptic e.g., [Co(NH3) 4Cl2] + complexes. Example 1 : When excess of aqueous KCN is added to an aqueous solution of copper sulphate. (i) What is the coordination entity formed? (ii) Why there is no precipitate of copper sulphide when H2S(g) is passed through this solution? Solution : CuSO4 + 4KCN  K2[Cu(CN)4] + K2SO4 (i) Thus, coordination entity formed = [Cu(CN)4] 2–. (ii) The complex ion is stable and does not dissociate (ionize) to give Cu2+ and CN– ions. Hence, no precipitation with H2S(g) is formed. CLASSIFICATION OF LIGANDS 1. On the Basis of Charge on Ligands (i) Anionic ligands (negatively charged ligands) e.g., F–, Cl–, CN–, S2–, SO4 2– etc. (ii) Neutral ligands (uncharged and electron pair donor) e.g., CO, NH3, H2O etc. (iii) Cationic ligands (positively charged ligands) e.g., NO+, NH – NH 2 3 + etc. 2. On the Basis of Denticity Denticity : It is number of coordinate bonds formed by one ligand. (i) Monodentate : Ligands having single donor atom, e.g., Cl–, H2O, NH3, NH – NH 2 3 + , etc. (also called unidentate ligands). (ii) Bidentate : Ligands can bind through two donor atoms (used simultaneously), e.g., CH – NH 2 2 CH – NH 2 2 C C O O– O O– C O O– CH2 NH2 en (ethylene-1, 2-diamine) ox (oxalate) gly (glycinate) N N 2, 2 - dipyridine (dipy) CH – C 3 CH – C 3 N – O– N – O – H Dimethylglyoxime (dmg) N N 1, 10-phenanthroline (phen) CH – C = CH – C – CH 3 3 O– O Acetylacetonate (acac)