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Coordination Complex

Stable inorganic or ionic compounds can be classified as simple compounds and complex compounds. Simple compounds are those which are formed by two or more elements and can be bifurcated as positive and negative ions in the dissolved state or in the solution phase.

The complex compounds are further divided into double salts and complex compounds or co ordination compounds. These are addition compounds formed by the combination of two or more stable compounds in stoichiometric proportions. Carnallite is a double salt formed when KCl and MgCl2 combine with 6molecules of water.(KCl.MgCl2.6H2O).
K2SO4 + Al2(SO4)3 + 24H2O forms potash Alum K2SO4.Al2(SO4)3. 24H2O. These are examples of double salts. In a dissolved state, these double salts lose their identity to form metal ions and non metal ions or radicals. CuSO4 combines in stoichiometric proportions with NH3 and water and forms tetrammine copper(II) sulphate mono hydrate (CuSO4.4NH3.H2O), which is a complex or co ordinate compound.

In a solution this will not completely dissociate and keep its identity. In solution the dissociation is Cuprammonium ion ([Cu(H2O) (NH3)4] 2+ and sulphate SO42-ions. In this topic, we will study some salient properties and the nomenclature of these complex or co ordinate compounds.

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Structure of the Coordination Complexes

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  1. Ions like BF4-, Ag(NH3)2+, Fe(CN)63- have distinctive properties that are quite unlike their associate molecules or ions. Because of their electronic structures, transition elements form a large number of Complex compounds.
  2. The presence of vacant orbitals in the outermost orbit and the easy accessibility of hybridization give the transition elements the unique property of forming the coordination bonds and complex compounds.
  3. A major part of modern research is in the field of study of the properties of the complex compounds and its uses.
  4. A Complex ion or compound consists of a central atom closely surrounded by a number of atoms or molecules that show a property of electron donation to the central atom.
  5. The central atom is generally called a nuclear atom and the surrounding groups are called ligands or coordinating groups. The oxidation state of the central atom is indicated in roman numerals in parentheses.
  6. The first or inner coordination sphere provides the nearest neighbor atoms to the nuclear atom. The number of atoms in the first coordination sphere is the coordination number of the nuclear atom.
  7. The distinguishing feature of a complex compound from other compounds is that both the central nuclear atom and the ligands are capable of independent existence as chemical species.
  8. Nuclear atom will generally be electron deficient and the ligands in a complex compound are electron donors.
  9. Complexes with coordination numbers 2 to 9 are known. But most compounds are 2,4,6 fold coordination. Two fold coordination compounds will exhibit a linear structure. The common examples are Cu(CN)2-, Ag(NH3)+2, Au(CN)2-, and Hg(NH3)2+2.
  10. Fourfold coordination exhibits tetrahedral geometry but it is mostly in the non transition element involved complexes like Cd(CN)4 2-, Hg(CN)42- and Zn(CN)42-. Pt, Pd,Cu and Ni (in their bivalent state) complexes show square planar geometry with coordination number four.
Six fold coordination is most common and occurs only in one geometric form which is the octahedron. With six fold coordination, the complexes exhibit both positional and optical isomerism in various compounds.

Applications of Coordination Complexes

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Coordination complexes are used as catalysts in many organic synthesis. Ruthenium chloride complex with di phosphine and di ammine is a catalyst used in hydrogenation and enentio selective hydrogenation reactions.

Complexes of Rh and Pt with Zn are common catalysts for hydro formylation reactions. Palladium complexes are used in Carbonylation and methyl carbonylation reactions. Cross coupling reaction of c-c bond, in macromolecular organic compounds are favored with complex compound catalysts.

Coordination complexes are used as precipitants of metals. The complex Nickel DMG , EDTA precipitation with Co,Fe etc. are examples of precipitating complexes. While some coordination complex compounds are soluble complexes and are used to extract the metals, Ag and Au are extracted as their sodium cyanide complexes which are soluble in water. Coordination complexes also find extensive use in bio medical applications. Some of the complex compounds are fluorescent and they are used to detect tumors in the body. They are used in the detection as well as treatment of cancer.

Analytically, coordination compounds find extensive use. EDTA is one of the very important reagents. The complex of Ferro and Ferri cynides are used as external indicators in volumetric analysis.

Nomenclature of Coordination Complexes

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Initially when there were not many complex compounds known, common names were used to represent them.
For example: Ferrocyanide ion Fe(CN)64-.

However with a number of new compounds getting synthesized rapidly, it became necessary to adopt a systematic naming procedure. Sufficient number of coordination complex compounds can be addressed by following a set of rules.
  1. Ligands are given special names. The anionic ligands end in 'o' . The neutral ligands will have the name of the molecule used. Water, ammonia, carbon monoxide and nitric acid are some exceptions and are given different names. Some of the names of ligands are given in the table below.
  2. Ligands are given first while naming the coordination complex. Greek prefixes like di, tri, tetra etc. are added to indicate the number of identical ligands present.
  3. The name of the central atom is given next, followed by its oxidation state given in Roman numerals in parentheses.
  4. The name of the central atom remains unchanged if the complex is a neutral molecule or a cation while the name of the central atom ends with 'ate' if the complex ion is negative.
  5. Prefixes 'bis' (2) 'tris(3)are used for complicated ligands instead of di and tri designation.
Some names of the coordinate complex ions and compounds.
  • Diamminesilver(I) ion Ag(NH3)2+
  • Tetramminezinc(II) ion Zn(NH3)42+
  • Triamminetrinitrocobalt(III) [Co(NH3)3(NO2)3]
  • Hexachloroplatinate(IV)ion PtCl62-
  • Hexacyanoferrate(II)ion Fe(CN)64-
  • Hexacyanoferrate(III)ion Fe(CN)63-
  • bis(thiosulfato)argentate(III)ion Ag(S2O3)23-
H2O Aqua
SO42- Sulfato

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