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Coordination compounds : Coordination compounds have been recognized by chemists as a special type of compound since late in the eighteenth century . Throughout the Nineteenth century , a wide variety of "complex inorganic compounds" were prepared by reacting metal salts with neutral organic or inorganic molecules, such as ethylene or ammonia, or with halide Ions and other simple anions, such as cyanide and nitrite . In 1892 Alfred Werner, proposed the Coordination Theory. 1 Werner's Coordination Theory : 1. In coordination compounds, central metal atoms exhibit primary valence and secondary valence. primary valence is ionic . Secondary valence is not ionic. The primary valence corresponds to oxidation state. The secondary valence corresponds to coordination number. 2. Every metal atom has a fixed number of secondary valences (coordination numbers). 3. The metal atom tends to satisfy both its primary valence as well as its secondary valence. Primary valence is satisfied by negative ions (metal ion has a positive charge) whereas secondary valence (coordination number) is satisfied either by negative ions or by neutral molecules). 4. The coordination number or secondary valences are always directed towards the fixed positions in space and this leads to definite geometry of the coordination compound 2 A coordination compound is one in which a central atom or ion is joined to one or more ligands through what is called a coordinate covalent bond in which both of the bonding electrons are supplied by the ligand. In such a complex the central atom acts as an electron-pair acceptor (Lewis acid) and the ligand as an electron-pair donor (Lewis base ). The central atom and the ligands coordinated to it constitute the coordination sphere. Thus the salt [Co(NH3)5Cl]Cl2 is composed of the complex ion [Co(NH3)5Cl]2+ and two Cl– ions; components within the square brackets are inside the coordination sphere, whereas the two chloride ions are situated outside the coordination sphere. These latter two ions could be replaced by other ions such as NO3– . 3 The central atoms of coordination complexes are most often cations (positive ions), but may in some cases be neutral atoms, as in nickel carbonyl Ni(CO)4. Ligands composed of ions such as F– or small molecules such as H2O or CN– possess more than one set of lone pair electrons, but only one of these pairs can coordinate with a central ion. Such ligands are said to be monodentate (―one tooth‖.) Larger ligands may contain more than one atom capable of coordinating with a single central ion, and are described as polydentate. Thus ethylenediamine (shown below) is a bidentate ligand. Polydentate ligands whose geometery enables them to occupy more than one coordinating position of a central ion act as chelating agents and tend to form extremely stable complexes known as chelates. Coordination sphere: The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere. For example: in the complex K4[Fe(CN)6]،[is the coordination sphere [Fe(CN)6]4- . Counter ions: The ions present outside the coordination sphere are called 4 counter ions. For example: in the complex K4[Fe(CN)6], K+ is the counter ion. A complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. A complex may also be called a coordination compound or metal complex. The ions or molecules surrounding the metal are called ligands. A ligand that is bound to a metal ion is said to be coordinated with the ion. The process of binding to the metal ion with more than one coordination site per ligand is called chelation. Compounds that bind avidly to form complexes are thus called chelating agents (for example, EDTA). Simple ligands like water or chlorine form only one link with the central atom and are said to be monodentate. Some ligands are capable of forming multiple links to the same metal atom, and are described as bidentate, tridentate etc. EDTA is hexadentate, which accounts for the great stability of many of its complexes. 5 Donor atom: An atom of the ligand attached directly to the metal is called the donor atom. For example: in the complex K4[Fe(CN)6 ] carbon is a donor atom. Coordination number: The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to Get the Power of Visual Impact on your side which the metal is directly bonded. For example: in the complex K4[Fe(CN)6], the coordination number of Fe is 6. 6 Naming Coordination Compounds: The coordination compounds are named in the following way. A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always name the cation before the anion. B. In naming the complex ion: 1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note: The metal atom or ion is written before the ligands in the chemical formula. 2. The names of some common ligands are listed in Table 1. For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g.sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide -o; -ate -ato; -ite -ito For neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine). Important exceptions: water is called ‗aqua‘, ammonia is called ammine‘, carbon monoxide is called carbonyl‘, and the N2 and O2 are called dinitrogen‘ and dioxygen‘. 7 8 2. Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead. The numerical prefixes are listed in Table 2. 9 3. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum.. If the complex ion is an anion, the name of the metal ends with the suffix –ate.. For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate). 10 4. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral in parentheses. C. To name a neutral complex molecule, follow the rules of naming a complex cation. Remember: Name the (possibly complex) cation BEFORE the (possibly complex) anion. 11 [Co(NH3)6]3+ Hexammine cobalt(III) K3[Fe(CN)6] potassium hexacyano ferrate(III) [Co(NH3)4Cl2]Cl Tetrammiine dichlorocobalt(III) chloride [FeNO(NH3)5]Cl3 Pentammine nitrosyl iron(II) chloride [Cr(CO)6] hexacarbonylchromium(0) K[V(CO)6] potassium hexacarbonyl vanadate(I) Q/name the following complexes : [Mn(NO)3CO] , NH4[Cr(NH3)2(NCS)4] , [Cu(en)2(H2O)2]SO4 12 1. Do ligands act like Lewis acids or Lewis bases? Why? 2. Do ligands form ionic bonds with the central metal atom? 3. What are chelating agents? 4. What is a monodentate ligand? 5. Describe polydentate ligands and provide an example. 6. What are hexadentate ligands? 7. Name this complex [Cu(NH3)4]SO4. 8. Name this complex [Co(en)3](NO3)2. 13 Q /For the complex K3[Cr(C2O4)3]·3H2O 1) What is the central metal ion? 2) What is its oxidation state? 3) What is its electronic configuration? 4) What is its coordination number? 5) What is the IUPAC name of the complex? 14 Q/Name the following compounds Answers: [Cu(OH)4] 2tetrahydoxocopper(II) Na3AuCl4 Sodium tetrachloroaurate(I) [Mo(CN)6 ]4hexacyanomolybdenate(II) Fe(CO)6ScCl6 hexacarbonyliron(III) hexachloroscanadate(III) [Cr(NH3)3(H2O)3 ]Cl3 triamminotriaquachromium(III) chloride [Pt(NH3)5Cl]Br3 pentaamminochloroplatinum(IV) bromide [Pt(en)2Cl2]Cl2 dichlorobis(ethylenediamine)platinum(II) chloride [Co(en)3]2(SO4)3 tris(ethylenediamine)cobalt(III) sulfate K4[Fe(CN)6] Potassium hexacyanoferrate(II) Na2[NiCl4] Sodium tetrachloronickelate(II) Pt(NH3)2Cl4 diamminoplatinum(IV) chloride (NH4)2[Ni(C2O4)2(H2O)2] Ammonium diaquabis(oxalate)nickelate(II) [CoBr(NH3)5]SO4 pentamminobromocobalt(III) sulfate [Fe(NH3)6][Cr(CN)6] hexamminoiron(III) hexacyanochromate(III) [Co(SO4)(NH3)5]+ pentamminosulfatocobalt(III) 15