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Coordination Compounds Coordination compounds CHAPTER COORDINATION COMPOUNDS 10 LEARNING OBJECTIVES (i) Appreciate the postulates of Werner’s theory of coordination compounds. (ii) Know the meaning of the terms: coordination entity, central atom/ion, ligand, coordination number, coordination sphere, coordination polyhedron, oxidation number, homoleptic and heteroleptic. (iii) Learn the rules of nomenclature of coordination compounds. (iv) Write the formulas and names of mononuclear coordination compounds. (v) Define different types of isomerism in coordination compounds. (vi) Understand the nature of bonding in coordination compounds in terms of the Valence Bond and Crystal Field theories. (vii) Learn the stability of coordination compounds. (viii) Appreciate the importance and applications of coordination compounds in our day to day life. INTRODUCTION Coordination compounds are a special class of compounds in which the central metal atom is surrounded by ions or molecules beyond their normal valency. These compounds are also referred to as complex compounds or simply complexes. In the modem terminology, these compounds are called coordination compounds. These compounds are widely present in the minerals, plants and animals, and play many important functions. Many biologically important compounds are coordination compounds in which complicated organic species are bound to the metal ions. The common examples are: haemoglobin, which is a coordination compound of iron, chlorophyll, which is a coordination compound of magnesium, Vitamin B12 which is a coordination compound of cobalt etc. The coordination compounds are finding extensive applications in metallurgical processes, analytical chemistry and medicinal chemistry. Many complex metal oxides and sulphides which constitute minerals are solid-state coordination compounds. MOLECULAR OR ADDITION COMPOUNDS : When solutions containing two or more simple stable compounds in molecular proportions are allowed to evaporate, crystals of new substances are obtained. These substances are termed molecular or addition compounds. Some common examples are as follows. Simple stable compounds Addition or molecular compound CuSO4 + 4NH3 CuSO4.4NH3 AgCN + KCN KCN.AgCN 4KCN + Fe(CN)2 Fe(CN2).4KCN K2SO4 + Al2 (SO4)3 + 24H2O K2SO4. Al2(SO4)3.24H2O Alum The molecular or addition compounds are of two types. Double salts or lattice compounds and coordination or complex compound. (A) Double salts or lattice compounds : The addition compounds which are stable in solid state only but are broken down into individual constituents, when dissolved in water are called double salts or lattice compounds.Their solution have the same properties as the mixture of individual compounds. For example when carnallite (KCl. MgCl2. 6H2O) is dissolved in water it 2+ + exhibit the properties of KCl and MgCl2. Mohr’s salt [FeSO4.(NH4)2SO4.6H2O] when dissolved in water gives Fe , NH4 and 2– SO4 ions in the solution, which gives the tests of all these ions. (B) Coordination or complex compounds : The addition compounds in which some of the constituent ions or molecules lose their identity and when dissolved in water they do not break up completely into individual ions are called coordination compounds. The properties of their solutions are different than those of their constituents. In such compounds there is complex ion which is a central metal ion with lewis bases attached to it through coordinate covalent bonds. On the basis of stability of complex ion, complex ions are further divided as follows. (a) Perfect complexes : Those in which complex ion is fairly stable and is either not dissociated or feebly dissociated in solution + 4– state, Ex. K4 [Fe(CN)6] 4K + [Fe(CN)6] Fe2+ + 6CN– (feebly dissociated) 4– The ferrocyanide ion [Fe(CN)6] is so in significantly dissociated that it can be considered as practically undissociated and does not give the test of Fe2+ or CN¯ ions. Gyaan Sankalp 1 Coordination compounds (b) Imperfect complexes: Those in which complex ion is less stable and is reversibly dissociated to give enough simple ions and thus imparts their tests, Ex. + 2– K2[Cd(CN)4] 2K + [Cd(CN)4] Cd + 4CN– (appreciably dissociated) An imperfect complex may be too unstable to exist and may be completely dissociated in solution, it then becomes a double salts. Cl WERNER’S COORDINATION THEORY HN3 NH Alfred Werner (considered as the father of coordination chemistry) studied 3 the structure of coordination complexes such as CoCl3.6NH3 and CuSO4. 4NH3 in 1893. According to him. (i) Each metal in coordination compound possesses two types of valencies : HN3 Co NH3 (a) Primary valency or principal valencies or ionisable valencies. (b) Secondary valency or non ionisable valencies. Cl (ii) Primary valency are satisfied by anions only. The number of primary valencies NH NH3 depends upon the oxidation state of the central metal. It may change from one 3 Cl compound to other. These are represented by dotted lines between central metal atom and anion [Co(NH3 ) 6 ]Cl 3 (iii) Secondary valencies are satisfied only by electron pair donor, the ions or the neutral species. These are represented by thick lines. (iv) Each metal has a fixed number of secondary valencies also referred as coordination number. The coordination number depends mainly on the size and the charge on the central atom. The maximum number of ions or molecules that the central atom can hold by secondary valencies is known as coordination number. (v) The ion attached to primary valencies possess ionising nature whereas the ions attached to secondary valencies do not ionise when the complex is dissolved in a solvent. (vi) Every central ion tends to satisfy its primary as well as secondary valencies. (vii) The secondary valencies are directional and are directed in space about the central metal ion. The primary valencies are non- directional. The presence of secondary valencies given rise to stereoisomerism in complexes. Initially, Werner had pointed out coordination number of a metal atom to be four or six. The six valencies were regarded to be directed to the corners of a regular octahedron circumscribed about the metal ion. For metals having four coordination number, the four valencies are either arranged in a planar or tetrahedral nature. Thus on the basis of werner theory, the CoCl3. 6NH3 is called hexaminecobalt (III) chloride because there are six ammonia ligands and the cobalt is in the +3 oxidation state, i.e. cobalt has three primary valencies and six secondary valencies. Now, it has been proposed that coordination number of a metal may be any whole number between 2 and 9. Definitions of some important terms pertaining to coordination compounds : (A) Central ion : (Centre of coordination) (i) The cation to which one or more neutral molecules or anions are attached is called as centre of coordination. (ii) Since the central ion acts as an acceptor and thus has to accommodate electron pairs donated by the donor atom of the ligands, it must have empty orbitals. (iii)This explains why the transition metals having empty d-orbitals form coordination compounds very readily. 2+ 3– 2+ 3+ (iv) In the complexes [Ni(NH3)6] and [Fe(CN)6] , Ni and Fe respectively are the central ions. (B) Ligand : (i) The neutral molecules, anions or cations which are directly linked with the central metal atom or ion in a complex ion are called 2+ 3+ ligands. For example in [CoCl(NH3)5] ligand is NH3, central ion is Co . (ii) Ligand donate a pair of electrons to the central atom. (iii)Ligands are attached to the central metal ion or atom through coordinate bonds or dative linkage. .. – – – (iv) Ligands are normally polar molecules like NH3 ; HO:2 or anions much as Cl , OH , CN etc. which contain atleast one ensured pair of valency electrons. (v) With few exceptions free ligands have lone pair of electron that is not engaged in bonding for example .. , :CN: : Cl.. : (vi) Ligands may be termed as Lewis base and central ion as Lewis acid. Types of ligands on the basis of number of donor atoms : Mono or unidentate ligands : They have one donor atom, i.e. they supply only one electron pair to central metal atom or ion. Ex. Neutral molecule % H2O , ROH, NH3, CO, NO, R3P, C5H5N etc. – – – –2 – Anion % Cl , OH , CN , SO4 , C6H5 etc. + + Cation % NO , NH2.NH3 etc. 2 Gyaan Sankalp Coordination compounds Ligands are three types : (i) Cation ligands (ii) Anion ligands (iii) Neutral ligands Monodentate Ligands : 1. Negative Ligand 2. Positive Ligand S. No. Formula Name of ligand S. No. Formula Name of ligand – 01 X Halo Nitrosonium 01 NO 02 OH – Hydroxo 02 NO Nitronium – 2 03 CN Cyano NH Ammonium 04 O –2 Oxo 03 4 –2 Peroxo Cl+ Chloronium 05 O2 04 – .. 06 NH Amido Hydrazinium 2 05 NHNH2 3 07 S – – Sulphido – – – 08 N Nitrido 3. Neutral Ligand 09 P – – – Phosphido 10 CH COO – Acetato 3 1 H2O Aqua (Aquo) 11 NO – Nitrato 3 2 NH Ammine – Nitro 3 12 NO2 13 ONO – Nitrito 3 CO Carbonyl –2 Carbonato 14 CO3 4 CS Thio carbonyl 15 SO –2 Sulphato 4 5 NO Nitrosyl –2 Sulphito 16 SO3 – 6 17 CNS Thiocyanato NS Thionitrosyl 18 S O –2 Thiosulphato 2 3 7 C6H5N Pyridine (py) Bidentate ligands : Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. Some examples are : H H CH2 N O O N N CH2 N C C H H O O Ethylenediamine (en) Oxalate (ox) 1,10-Phenanthroline (–phen) H CH2 N H N N O C O C O O O Glycinato (Gly) 2,2'-Dipyridine (Dipy) Carbonato Gyaan Sankalp 3 Coordination compounds Tridentate ligands : The ligands having three donor atoms are called tridentate ligands. H H H N N H N N HC2 CH2 N HC2 N HC2 H Diethylene triamine (dien) 2,2',2''Terpyridine (terpy) Tetradentate ligands : These ligands possess four donor atoms.
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