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9., CO-ORDINATION COMPOUNDS
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•These are compounds in which a metal atom/ion is surrounded by, a group of anions or neutral molecules. The metal atom/ion is, called central atom/ion and the neutral molecules or anions are, called ligands., •The central atom/ion should contain vacant orbitals and the ligand, , should contain one or more lone pairs of electrons., •The central metal atom or ion and ligands form a co-ordination, entity., , •A ligand that binds to the central atom/ ion through a single donor, atom is said to be unidentate ligand. E.g.: Cl- , Br- , I- , OH- , H2O,, , NH3, CN- , NC- , SCN- etc
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•A ligand that binds to the central atom through two donor atoms is, , called a bidentate ligand., •E.g.: Ethane-1,2-diamine or ethylenediamine, (H2NCH2CH2NH2) notated as ‘en’ and oxalate ion (C2O4 2–)., • A ligand that binds to the central atom through more than two, donor atoms is called polydentate ligand., , •E.g.: Ethylenediamine tetraacetate ion (EDTA4–) is an, important hexadentate ligand., •The total number of ligand donor atoms to which the metal is, directly bonded is called the co-ordination number of the metal, atom/ion.
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IUPAC Nomenclature of Co-ordination Compounds:, The following rules are used while naming co-ordination compounds:, (i) The cation is named first in both positively and negatively charged coordination entities., (ii) The ligands are named in alphabetical order before the name of the, central atom/ion., (iii) Names of the anionic ligands end in –o, those of neutral and cationic, ligands are the same except ‘aqua’ for H2O, ‘ammine’ for NH3,, ‘carbonyl’ for CO and ‘nitrosyl’ for NO., (iv) Prefixes mono, di, tri, etc., are used to indicate the number of individual, ligands in the co-ordination entity. When the names of the ligands, , include a numerical prefix, then the terms bis (for 2 such ligands), tris, (for 3), tetrakis (for 4) are used. Here the name of the ligand is placed in, simple bracket.
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(v) Oxidation state of the metal is indicated by Roman numeral in, simple bracket., , (vi) If the complex ion is a cation, the central atom is named same, as the element. If the complex ion is an anion, the name of the, , metal ends with the suffix –ate. For example ferrate for iron,, cobaltate for cobalt, zincate for Zn etc., (vii) The neutral complex molecule is named similar to that of the, , cationic complex
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Isomerism in Co-ordination Compounds, ❑Compounds having the same molecular formula but different, , structural formula or spatial arrangement of atoms are called, isomers and the phenomenon is called isomerism., , ➢The isomerism shown by co-ordination compounds are broadly, divided into two – structural isomerism and stereo isomerism., I) Structural Isomerism These are isomers which differ in, , the structural arrangement of ligands around the central, atom. They are of four types:
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1) Ionisation Isomerism: It arises due to the inter change of ions, between the inside and outside of coordination sphere. They give, , different types of ions in aqueous solution., • An example is [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4., , 2) Linkage isomerism: It arises in a co-ordination compound, containing ambidentate ligand, which can bind to the central atom, through more than one donor atoms., , • E.g. in the complex [Co(NH3)5(ONO)]Cl2, the nitrite ligand is, bound through oxygen (–ONO), and in [Co(NH3)5(NO2)]Cl2, the nitrite ligand is bound through nitrogen (–NO2) atom.
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3) Co-ordination Isomerism: If both anionic and cationic parts are, , complexes, the isomerism arises due to the interchange of ligands, between cationic and anionic entities. This type of isomerism is, called coordination isomerism., •An example is [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]., , 4) Solvate isomerism: It arises due to the difference in the no. of, , solvent molecules which are directly bonded to the metal ion as, ligand. It is also known as ‘hydrate isomerism’ if water is the, solvent., , •An example is [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2.H2O
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II) Stereoisomerism:, , These are isomers which differ only in the spatial arrangement of, ligands around the central atom. They have same atom to atom, bond., •These are of two types:, (i) Geometrical isomerism (ii) Optical isomerism, , i), , Geometrical Isomerism:, , This type of isomerism arises due to the difference in the, arrangements of the ligands around the central atom. It is mainly, found in co-ordination complexes with co-ordination numbers 4, (square planar complexes) and 6 (octahedral complexes).
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Geometrical isomer in which the same ligands are on the same side of, , the central metal atom is called cis isomer and the isomer in which the, same ligands are on the opposite side is called trans isomer., •Square planar complexes with formula [MX2L2] (X and L are, unidentate ligands) can show this isomerism. E.g.: [Pt (NH3)2Cl2]
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Octahedral complexes with formula [MX2L4] can also show this type of, isomerism. e.g.: [Co(NH3)4Cl2], , This type of isomerism also arises when bidentate ligands (L – L) are present in, complexes with formula [MX2(L – L)2] e.g.: [Co (en)2Cl2]+
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ii) Optical Isomerism:, •Optical isomers are mirror images that cannot be superimposed on, one another. These are also called enantiomers., •There are two forms of optical isomers - dextro (d) and laevo (l)., • In a co-ordination entity of the type [PtCl2(en)2]+ , only the cisisomer shows optical activity., •The trans- isomer has a plane of symmetry and is optically, inactive.
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Magnetic Properties of Co-ordination Compounds, , •By knowing the magnetic moment, we can predict the geometry of, complexes., •For e.g. [Mn(CN)6]3– has magnetic moment of two unpaired, electrons while [MnCl 6] 3– has a paramagnetic moment of four, unpaired electrons., , •[Fe(CN)6]3– has magnetic moment of a single unpaired electron, while [FeF6]3– has a paramagnetic moment of five unpaired, electrons., •[CoF6]3– is paramagnetic with four unpaired electrons while, [Co(C2O4) 3]3– is diamagnetic., , •This is because [Mn(CN)6]3–, [Fe(CN) 6]3– and [Co(C2O4)3]3–, are inner orbital complexes involving d 2 sp3 hybridisation., • While [MnCl6]3– , [FeF6]3– and [CoF6] 3– are outer orbital, complexes involving sp3 d2 hybridisation and are paramagnetic.
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Bonding in Metal Carbonyls, Transition metals form a large number of homoleptic carbonyls. Eg., [Ni(CO)4], [Fe(CO)5], [Cr(CO)6], [Co2(CO)8], [Mn2(CO)10] etc., These carbonyls have simple, well defined structures.
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•The metal-carbon bond (M – C bond) in metal carbonyls possess, both s and p character., , •The M – C bond is formed by the donation of lone pair of, electrons on the carbonyl carbon into a vacant orbital of the metal., •The M–C bond is formed by the donation of a pair of electrons, from a filled d orbital of metal into the vacant anti-bonding * orbital, of carbon monoxide., •Thus the metal to ligand bonding creates a synergic effect which, strengthens the bond between CO and the metal.
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Application of Co-ordination Complexes, 1. In Qualitative & Quantitative Analysis: Ni2+ is detected and, estimated by the formation of a complex with Dimethyl Glyoxime, (DMG)., •, , The brown ring test for the detection of nitrate ion is due to the, formation of the brown complex [Fe(H2O)5NO]2+ ., , 2. In water treatment: The Hardness of water is estimated by simple, titration with Na2EDTA (sodium salt of EDTA)., , •The hardness of water can be removed by the formation of a, complex with calgon., 3. In Metallurgy for the extraction of some metals like Ag & Au and, for refining of metal like Ni., 4. Biological Applications: Chlorophyll, is a co-ordination compound, of magnesium, Haemoglobin, is a coordination compound of iron and, Vitamin B12 is a co-ordination compound of cobalt.
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5. In Catalysis: Co-ordination compounds are used as catalysts, for many industrial processes., 6. In electroplating, 7. In Photography, 8. In medicine: Cis-platin is used for the treatment of cancer., , EDTA is used in the treatment of lead poisoning.
