Page 1 : Chemistry-Mojo, Simplifying Chemistry, , Workbook for CBSE/ISC XII, , Co-ordination (Complex) Compounds, Day-1, The chemistry of coordination compounds is an important and challenging area of, modern inorganic chemistry., New concepts of chemical bonding and molecular structure have provided insights, into the functioning of vital components of biological systems., Chlorophyll, haemoglobin and vitamin B12 are coordination compounds of, magnesium, iron and cobalt respectively., Variety of metallurgical processes, industrial catalysts and analytical reagents involve, the use of coordination compounds., Coordination compounds also find many applications in electroplating, textile dyeing, and medicinal chemistry., , Co-ordination compounds / complex compounds :, A complex compound is a molecular compound that results from the combination of two or, more simple stable molecular compounds & retain its identity in the solid as well as in, solution., Complex ion : An electrically charged ion which consists of a central metal atom or ion, surrounded by a group of ions or neutral molecules., Differences between Double salt & Complex compound ., , Double Salt, 1. Exist only in solid state., 2. Ions present in constituents are obtained in, aqueous solution., 3. It loses identity in aqueous solution., 4. Contains ionic bond & metal ions show, their normal (primary) valency., 5. Properties of double salts are same as, those of constituent compounds., , Tel +91 84330 83279 , 9837603674,
[email protected], , Co-ordination compound, 1. Exist in solid state as well as in aqueous, solution., 2. Simple ions present in constituent salt are, not obtained in aqueous solution but new, complex ions are formed., 3. It does not completely lose its identity in, aqueous solution., 4. Contains ionic as well as co-ordinate bond., Metal ions show primary as well as secondry, valencies., 5. Properties of complex are different from, those of constituent compounds., , Page 1
Page 2 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , Coordination entity, A coordination entity constitutes a central metal atom or ion bonded to a fixed number, of ions or molecules., For example, [CoCl3(NH3)3] is a coordination entity in which the cobalt ion is, surrounded by three ammonia molecules and three chloride ions., Other examples are [Ni(CO)4], [PtCl2(NH3)2], [Fe(CN)6]4–, [Co(NH3)6]3+ ., Central atom/ion, In a coordination entity, the atom/ion to which a fixed number of ions/groups are, bound in a definite geometrical arrangement around it, is called the central atom or, ion., For example, the central atom/ion in the coordination entities: [NiCl2(H2O)4],, [CoCl(NH3)5]2+ and [Fe(CN)6]3– are Ni2+, Co3+ and Fe3+ , respectively., These central atoms/ions are also referred to as Lewis acids., , Study of ligands – Give definition & Examples, Ligands : - The neutral molecules, anions & cations which are directly linked with the central, ___________ atom or ion within a complex ion are called Ligands., Ligands are attached to central metal atom or ion through a co-ordinate bond or dative, linkage., Ligands act as a ______________ (electron pair donors) & Central metal atom act as Lewic, acid (electron pair acceptors)., , Types of Ligands :, a. Mono-dentate ligand : Donate only _______ electron pair to central atom., e.g H2O , NH3 , CO ,, b. Bi-dentate ligand : Ligands which possess two donor atoms & have ability to link with, central metal atom/ion at two positions are called bi-dentate ligand., e.g ethylenediamine (en) , Oxalate (ox), Glycinate (Gly) , carbonate, etc., c. Tri- , Tetra- & Penta- dentate ligand, Tri-dentate ligand – has three donor atoms. e.g Diethylenetriamine, Tetra-dentate ligand – has four donor atoms. e.g Triethylenetetramine(trien), Penta-dentate ligand – has five donor atoms. e.g Ethylenediaminetriacetate ion, d. Hexa-dentate ligand – has six donor atoms. e.g ethylenediaminetetraacetate (EDTA), e. Chelating Ligand – A bi-dentate or poly-dentate ligand which on co-ordination with, central metal atom or ion results in the formation of cyclic or closed ring structures are, known as chelating ligands., Tel +91 84330 83279 , 9837603674,
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Page 3 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , , , Simplifying Chemistry, , Complex thus formed are called Chelates., , Characteristics of Chelating Complex :, 1. Chelating ligands form more stable complex in comparison to mono-dentate ligands. This, is known as Chelating effect., 2. Chelates containing 5 or 6-membered rings including metal atom/ion are comparatively, more stable., 3. Ligands with larger groups form more stable rings than with the smaller groups due to, steric hindrance., Importance of Chelates, 1. In softening of hard water., 2. In separation of Lanthanoids & Actinoids., 3. Estimation of certain ionsdue to formation of stable & coloured chelates., 4. In food preservation., 5. In solvent extraction., 6. In the elimination of harmful radioactive metals from body. EDTA is used for this, purpose., Ambidentate ligands : Ligands which have two or more donor atoms but only one donor, atom is attached to central metal atom/ion. e.g NO2 , ONO , CN , NC , SCN, NCS, Flexi-dentate ligands : Poly-dentate ligands, it is not necessary that all donor atoms present, in polydentate ligand should form co-ordinate bond with central metal atom/ion., e.g EDTA which is hexa-dentate ligand can also function as penta-dentate or tetra-dentate, ligand with certain metal ions., , , Sulphate ion can also act as mono-dentate ligand., , Co-ordination number : The number of atoms of the ligands that are directly bonded to the, central metal atom/ion by co-ordinate bond is known as Co-ordination number., Co-ordination number = Number of ligands × Denticity of ligand., , , , For example, in the complex ions, [PtCl6]2– and [Ni(NH3)4]2+, the coordination, number of Pt and Ni are 6 and 4 respectively., Similarly, in the complex ions, [Fe(C2O4)3]3– and[Co(en)3]3+, the coordination number, of both, Fe and Co, is 6 because C2O42– and en (ethane-1,2-diamine) are didentate, ligands., , Tel +91 84330 83279 , 9837603674,
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Page 4 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , , , Simplifying Chemistry, , It is important to note here that coordination number of the central atom/ion is, determined only by the number of sigma bonds formed by the ligand with the central, atom/ion., Pi bonds, if formed between the ligand and the central atom/ion, are not counted for, this purpose., , Calculation for Co-ordination number of a complex coordination sphere., a. [Cu(NH3)4]SO4, b. [ Fe (EDTA)]2-, , c. K3[Fe(C2O4)3], , Oxidation state (number) of an element in a complex & its calculation., The oxidation number of the central atom in a complex is defined as the charge it, would carry if all the ligands are removed along with the electron pairs that are shared, with the central atom., The oxidation number is represented by a Roman numeral in parenthesis following, the name of the coordination entity. For example, oxidation number of copper in, [Cu(CN)4]3– is +1 and it is written as Cu(I)., Charge on complex ion = Oxidation number of complex ion + Charge on ligand, Examples, a. [Cu(NH3)4]SO4, b. [ Fe(CN)6]4c. K3[Fe(C2O4)3], d. [Ni(CO)4], e. [Cu(NH3)2]2+, f. [Cr(H2O)6]3+, , Tel +91 84330 83279 , 9837603674,
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Page 7 : Chemistry-Mojo, Simplifying Chemistry, , Workbook for CBSE/ISC XII, , 2. Naming of Ligands : Ligands name are discussed in ‘Classification of Ligands’., 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’., 3. Numerical prefixes to indicate number of ligands : If there are several ligands of same, type then prefix di, tri, tetra are used to indicate the number of ligands of that type., If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate, ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-,, pentakis-, are used instead., NiCl2 (PPh3)2] is named as dichloridobis(triphenylphosphine)nickel(II), 4. Order of naming ligands : Ligands are named in alphabetical order followed by name of, metal ion., 5. Naming of complex ion & ending of the central metal atom ., a. Cationic complex & Neutral complex : If the complex ion is a cation, the metal is named, same as the element., For example, Co in a complex cation is called cobalt and Pt is called platinum., , [Cr(NH3)3(H2O)3]Cl3, , :, , [Pt(NH3)5Cl]Br3, , :, , [Pt(H2NCH2CH2NH2)2Cl2]Cl2, , :, , [Co(H2NCH2CH2NH2)3]2(SO4)3, [Pt(NH3)2Cl4], [Fe(CO)5], b. Anionic complex ion : 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)., , K4[Fe(CN)6], Tel +91 84330 83279 , 9837603674,
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Page 9 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , (iii) Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first, letter of the abbreviation is used to determine the position of the ligand in the alphabetical, order., (iv) The formula for the entire coordination entity, whether charged or not, is enclosed in, square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses., Ligand abbreviations are also enclosed in parentheses., (v) There should be no space between the ligands and the metal within a coordination sphere., (vi) When the formula of a charged coordination entity is to be written without that of the, counter ion, the charge is indicated outside the square brackets as a right superscript with the, number before the sign. For example, [Co(CN)6]3–, [Cr(H2O)6]3+ , etc., (vii) The charge of the cation(s) is balanced by the charge of the anion(s). The names of, coordination compounds are derived by following the principles of additive nomenclature., Thus, the groups that surround the central atom must be identified in the name. They are, listed as prefixes to the name of the central atom along with any appropriate multipliers., Q. Write the formulas for the following coordination compounds:, (a) Tetraammineaquachloridocobalt(III) chloride, , (b) Potassium tetrahydroxidozincate(II), , (c) Potassium trioxalatoaluminate(III), , (d) Dichloridobis(ethane-1,2-diamine)cobalt(III), , (e) Tetracarbonylnickel(0), , (f) Tetraamminediaquacobalt(III) chloride, , (g) Potassium tetracyanidonickelate(II), , (h) Tris(ethane–1,2–diamine) chromium(III) chloride, , (i) Amminebromidochloridonitrito-N-platinate(II), , Tel +91 84330 83279 , 9837603674,
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Page 12 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , d. Co-ordination Isomerism : Definition with example, Compounds having same molecular formula & both Cationic & anionic complexes. But they, differ in the different distribution of ligands., [Co(NH3)6][Cr(CN)6] & [Cr(NH3)6][Co(CN)6], , [B]. Stereo Isomerism : Definition, Compounds having same molecular formula but different in the spatial arrangement of atoms, or groups, then such compounds are known as stereo isomer & phenomenon is called stereoisomerism., a. Geometrical Isomerism : Definition, Compounds having same molecular formula but differ in the relative position of ligands in, space. If ligands occupy position adjacent to each other, then cis, if opposite to each other,, then trans. This isomerism is also known as cis- trans isomerism., Square planar complexes of the types Ma4, Ma3b, Mab3 do not show geometrical isomerism., Square planar complexes of the type Ma2b2 can exist in cis & trans form., , In Square Planar Complex, i. Type MA2B2 complex e.g [Pt (NH3)2Cl2], , Tel +91 84330 83279 , 9837603674,
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Page 14 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , iii. Type MA3B3 e.g [RhCl3(py)3], , Optical Isomerism :, , , , , , , , Optical isomerism is of common occurence in octahedral complexes having coordination number six ., Co-ordination compound should possess at least one symmetrical di-dentate ligand, Optical isomers rotate the plane of polarized light in opposite directions ., Optical isomers are structurally non-superimposable images of each other which are, oftenly called enantiomers or enantiomorphs ., The non-superimposable isomers possess chirality or asymmetry ., Enantiomers are of two types i.e dextro isomer which rotate the plane of polarized, light to clockwise direction & laevo form which rotate the plane of polarized light in, anti-clockwise direction ., , Conditions to exhibit Optical Isomerism :, 1., , 2., , Examples, a. Type M(AB)3 e.g [Co(en)3]3+, , Tel +91 84330 83279 , 9837603674,
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Page 15 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , b. M(AA)2B2 e.g [CoCl2(en)2]+, This type of compound exhibit both geometrical & optical isomerism . Trans isomer does not, show optical activity due to its symmetrical structure but cis isomer being unsymmetrical, exhibit optical isomerism ., , c. M(AA)B2C2, , e.g [Co(en)(NH3)2Cl2], , Q Square planer complex do not show optical isomerism. Why ?, Ans. because they contain plane of symmetry ., Note : Tetrahedral complex containing unsymmetrical bi-dentate ligand are optically active ., e.g [Ni (CH2NH2COO)2] bis(glycinato)nickel(II), Q. Tetrahedral complexes do not exhibit Geometrical isomerism . Why ?, Ans. because all the four valencies are equivalent or symmetrical ., , Werner's Theory of Co-ordination compounds, Main Postulates :, 1. Metal Possess two types of valencies called, a. Primary Valencies (Oxidation state), , , Valency which metal exhibit during the formation of simple salts., , b. Secondry Valencies (Co-ordination number), Valency which a metal atom or cation offer to neutral molecules or negative ions, (Ligands) in the formation of complex entity., Every metal cation has fixed number of secondry valencies., , Tel +91 84330 83279 , 9837603674,
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Page 16 : Chemistry-Mojo, Simplifying Chemistry, , Workbook for CBSE/ISC XII, , 2. ______________valencies are satisfied by negative ions while _______________, valencies are satisfied by negative ions & neutral molecules called ligands ., 3. Primary valencies being ionic are non-directional in nature but secondry valencies being, co-ordinate are directional in nature , therefore they contribute towards geometrical structure, of complex ., , Day-4, , Valence bond theory of coordination compounds –, Main Postulates of Valence bond theory are :, 1. _____________ bond exist between metal & ligand by donation of electrons ., 2. To accommodate these electrons metal atom must possess required number of, vacant orbitals having equal energy ., 3., 4., 5., 6 .Resulting complex acquire shape based on hybridization of central metal atom or ion ., Hybridization, sp3, dsp2, sp3d, sp3d2, d2sp3, , Shape, , Octahedral Complex, Examples of formation of, 1. Inner orbital Complex (Definition), When the complex formed involves the inner (n-1)d orbital for hybridization (d2sp3) , the, complex is called Inner orbital complex or hyperligated complex., Such type of complex have lesser number of unpaired electrons, therefore they are known as, __________ (high/low) spin complex., , Tel +91 84330 83279 , 9837603674,
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Page 17 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , a. [Fe(CN)6]3Electronic config of Metal, in ground state, Electronic config, Of metal ion, , Hybrid orbitals of, Metal ion, Formation of, complex ion, Hybridization _______________ Magnetic Character _____________________ ., b. [Co(NH3)6]3+, Electronic config of Metal, in ground state, Electronic config, Of metal ion, , Hybrid orbitals of, Metal ion, Formation of, complex ion, Hybridization _____________ Magnetic Character _____________________, , b. Outer orbital Complex (Definition), , When the complex formed involves the outer nd orbital for hybridization (sp3 d2) , the, complex is called Outer orbital complex or hypoligated complex., Such type of complex have higher number of unpaired electrons, therefore they are known as, __________ (high/low) spin complex., a. [CoF6]3Electronic config of Metal, in ground state, Tel +91 84330 83279 , 9837603674,
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Page 18 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , Electronic config, Of metal ion, , Hybrid orbitals of, Metal ion, Formation of, complex ion, , Hybridization ___________________ Magnetic Character _____________________, b. [Fe(H2O)6]2+, Electronic config of Metal, in ground state, Electronic config, Of metal ion, , Hybrid orbitals of, Metal ion, Formation of, complex ion, , Hybridization __________________ Magnetic Character _____________________, , Prediction of Magnetic Character on the basis of Valence bond theory :, Geometry of complex can be predicted if its magnetic moment is known i.e, number of unpaired electrons is known., [NiCl4]2- : (2 unpaired electrons), , Tel +91 84330 83279 , 9837603674,
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Page 19 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , [Ni(CN)4]2- : (Zero unpaired electrons), , Limitations of Valence Bond Theory, 1., 2., 3., 4., 5., , It cannot distinguish between strong & weak field ligands ., No satisfactory explanation for colour of the complex ., Cannot give correct explanation of thermodynamic & kinetic stability of complex ., Magnetic character predicted by valence bond theory is not always exact ., It cannot explain why some complexes of a metal ion in a prticular oxidation state are, low spin while some other complexes of the same metal ion in the same oxidation, state are high spin ., , Q. Why is complex [Cu(NH3)4]2+ square planar & not tetrahedral ?, Ans, , Day-5, , Crystal Field Theory (CFT), The valence bond theory could not explain the stability of coordination compounds. It also, failed to throw a light on the differences between strong and weak ligands. Therefore,, scientists proposed the crystal field theory., According to this theory, the metal-ligand bond acts as an ionic bond arising purely from the, electrostatic interactions between the metal ions and ligands. This theory takes anions as, point charges and neutral molecules as dipoles. When the transition metals do not bond to any, ligand, their d orbitals degenerate. This means that they have the same amount of energy., When they start bonding with other ligands, the d orbitals split apart and become nondegenerate. This bonding occurs mainly due to different symmetries of the d orbitals and the, inductive effect of the ligands on the electrons., The pattern of the splitting of d orbitals depends on upon the nature of crystal field., (i) Octa-hedral crystal field, (ii) Tetrahedral crystal field, (iii) Square-planar crystal field., Tel +91 84330 83279 , 9837603674,
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Page 20 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , Crystal field theory, Splitting of Crystal Fields, , , , , , , , , In case of an octahedral coordination compound, there are six ligands that surround, the metal atom/ion., In these cases, we observe repulsion between the electrons in d orbitals and ligand, electrons., This repulsion is more in the case of dx2-y2 and dz2 orbitals because they point towards, the axes along the direction of ligand., Hence, their energy is higher as compared to the average energy in spherical crystal, field., While, dxy, dyz and dxz orbitals experience lower repulsions as they are directed, between the axes., Hence, these three orbitals possess lower energy than the average energy in spherical, crystal field., Thus, we get two energy levels:, t2g– set of three orbitals (dxy, dyz and dxz) with lower energy, eg – set of two orbitals (dx2-y2 and dz2) with higher energy, , Crystal field splitting in Octahedral Complex. (Diagramatic Representation), , Cryatal Field Splitting energy (Crystal field stablising energy) (Δ˳), (Definition), , Tel +91 84330 83279 , 9837603674,
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Page 21 : Chemistry-Mojo, Simplifying Chemistry, , Workbook for CBSE/ISC XII, , Main postulates of Crystal field theory are :, 1., The crystal field theory considers the bond between Metal & Ligand to be ionic ., 2., Theory treats each ligand as a point of negative charge ., 3., It considers the effect of ligands on the relative energies of d-orbitals of the central, metal atom or ion ., 4., The pattern of splitting depends upon the nature of the crystal field exerting its, influence on the central metal / ion ., Spectro-chemical series :, I- < Br- < S2- < SCN- < Cl- < F- < OH- < C2O42- < O2- < H2O < NCS- < NH3 < en < NO2- < CN- < CO, , Weak field ligands, , [Value CFSE increase →], , Strong field ligands, , Crystal Field splitting in Tetrahedral complex. (Diagramatic Representation), , Explanation of the properties of Complex by Crystal Field Theory :, a. High spin complex & Low spin complex, Strong Field Ligand →, Weak Field Ligand →, , High CSFE value, Low CSFE value, , →, →, , _______ spin complex (Δo > P), _______ spin complex (Δo< P), , Δo = Crystal field splitting energy ., P = Pairing energy i.e energy required for electron pairing in a single orbital ., b. Colour :, Transition metal complex absorb visible light which excites electron from lower, d-orbital to the higher ones , this justifies that the CFSE (Δo) for most of the, complexes is of same order as provided by photons of visible light . The color of, complex will be the transmitted color which is the complementary color to that of, absorbed color ., One of the most distinctive properties of transition metal complexes is their wide, range of colours., Tel +91 84330 83279 , 9837603674,
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Page 22 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , , , , , , , , , , , , , , , Simplifying Chemistry, , This means that some of the visible spectrum is being removed from white light as it, passes through the sample, so the light that emerges is no longer white., The colour of the complex is complementary to that which is absorbed., The complementary colour is the colour generated from the wavelength left over; if, green light is absorbed by the complex, it appears red., The colour in the coordination compounds can be readily explained in terms of the, crystal field theory., Consider, for example, the complex [Ti(H2O)6]3+, which is violet in colour. This is an, octahedral complex where the single electron (Ti3+ is a 3d1 system) in the metal d, orbital is in the t2g level in the ground state of the complex. The next higher state, available for the electron is the empty eg level., If light corresponding to the energy of blue-green region is absorbed by the complex,, it would excite the electron from t2g level to the eg level (, , →, , )., Consequently, the complex appears violet in colour ., The crystal field theory attributes the colour of the coordination compounds to d-d, transition of the electron., It is important to note that in the absence of ligand, crystal field splitting does not, occur and hence the substance is colourless., For example, removal of water from [Ti(H2O)6]Cl3 on heating renders it colourless., Similarly, anhydrous CuSO4 is white, but CuSO4.5H2O isblue in colour., The influence of the ligand on the colourof a complex may be illustrated by, considering the [Ni(H2O)6]2+ complex,which forms when nickel(II) chloride is, dissolved in water., If the didentate ligand, ethane-1,2-diamine(en) is progressively added in the molar, ratios en:Ni, 1:1, 2:1, 3:1, the following series of reactions and their associated colour, changes occur:, [Ni(H2O)6]2+ (aq) + en (aq) = [Ni(H2O)4(en)]2+(aq) + 2H2O, green, (pale blue), [Ni(H2O)4(en)]2+(aq) + en (aq) = [Ni(H2O)2(en)2]2+ (aq) + 2H2O, (blue/purple), [Ni(H2O)2(en)2]2+ (aq) + en(aq) = [Ni(en)3]2+ (aq) + 2H2O, (violet), , c. Magnetic character (Magnetism), , , , , The transition metal complexes containing one or more unpaired electrons possess a, definite value of dipole moment., An unpaired electron because of its spin is equivalent to an electric current flowing in, a circular conductor. Hence, it behaves as a magnet., The magnetic nature or magnetic moment of a co-ordination compound depends on, number of unpaired electron., Magnetic moment =, , Tel +91 84330 83279 , 9837603674,
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Page 23 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , , , , , Simplifying Chemistry, , The above relationship is used to calculate the number of of unpaired electrons in a, complex ion., The actual number of of unpaired electrons in a complex can be found by magnetic, measurements., A distinction between low and high spin complexes can be made only if the metal ion, contains more than three and less than 8 d-electrons., The high spin complex [Fe(H2O)6]2+ has 4 unpaired electrons and is is, paramagnetic where else the low spin complex [Fe(CN)6]4- has no unpaired, electrons and is diamagnetic., , Limitation of Crystal Field Theory :, The crystal field theory is highly useful and more significant as compared to the valence bond, theory. Even after such useful properties, it has many limitations., The following points will clearly state the limitations of crystal field theory:, 1. The assumption that the interaction between metal-ligand is purely electrostatic cannot be, said to be very realistic., 2. This theory takes only d-orbitals of a central atom into account. The s and p orbits are not, considered for the study., 3. The theory fails to explain the behavior of certain metals which cause large splitting while, others show small splitting. For example, the theory has no explanation as to why H2O is a, stronger ligand as compared to OH–., 4. The theory rules out the possibility of having p bonding. This is a serious drawback, because p bonding is found in many complexes., 5. The theory gives no significance to the orbits of the ligands. Therefore it cannot explain, any properties related to ligand orbitals and their interaction with metal orbitals., , Stability of Co-ordination compounds :, Stability on the basis of magnitude of Stability constant (K), The stability of a complex in solution refers to the degree of association between the two, species involved in the state of equilibrium., The magnitude of the (stability or formation) equilibrium constant for the association,, quantitatively expresses the stability., Thus, if we have a reaction of the type:, M + 4L → ML4, then the larger the stability constant, the higher the proportion of ML4that exists in solution., Free metal ions rarely exist in the solution so that M will usually be surrounded by solvent, molecules which will compete with the ligand molecules, L, and be successively replaced by, them., , Tel +91 84330 83279 , 9837603674,
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Page 25 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , 4. Presence of Chelate rings : Formtion of chelate rings increases the stability of complex., The stabilization due to chelation is called Chelate effect – found maximum in five or six, membered rings)., 5. Effect of Multi-dentate cyclic ligands : If the ligand is multidentate & cyclic without any, steric effect, the stability of the complex is further increased. The effect is called Macrocyclic, effect., , Application of Co-ordination compounds in different fields., The coordination compounds are of great importance., These compounds are widely present in the mineral, plant and animal worlds and are known, to play many important functions in the area of analytical chemistry, metallurgy, biological, systems, industry and medicine., These are described below:, , , Coordination compounds in many qualitative and quantitative chemical analysis., The familiar colour reactions given by metal ions with a number of ligands (especially, chelating ligands), as a result of formation of coordination entities, form the basis for, their detection and estimation by classical and instrumental methods of analysis., Examples of such reagents include EDTA, DMG (dimethylglyoxime), α–nitroso–β–, naphthol, cupron, etc., , , , Hardness of water is estimated by simple titration with Na2EDTA., The Ca2+ and Mg2+ ions form stable complexes with EDTA., The selective estimation of these ions can be done due to difference in the stability, constants of calcium and magnesium complexes., , , , Some important extraction processes of metals, like those of silver and gold, make, use of complex formation., Gold, for example, combines with cyanide in the presence of oxygen and water to, form the coordination entity [Au(CN)2]– in aqueous solution. Gold can be separated in, metallic form from this solution by the addition of zinc ., , , , Similarly, purification of metals can be achieved through formation and subsequent, decomposition of their coordination compounds., For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield, pure nickel. (Mond’s Process), Coordination compounds are of great importance in biological systems., a. The pigment responsible for photosynthesis, chlorophyll, is a coordination, compound of magnesium., b. Haemoglobin, the red pigment of blood which acts as oxygen carrier is a, coordination compound of iron., , , , Tel +91 84330 83279 , 9837603674,
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Page 26 : Chemistry-Mojo, Workbook for CBSE/ISC XII, , Simplifying Chemistry, , c. Vitamin B12, cyanocobalamine, the anti– pernicious anaemia factor , is a, coordination compound of cobalt., d. Among the other compounds of biological importance with coordinated metal ions, are the enzymes like, carboxy-peptidase A and carbonic anhydrase (catalysts of, biological systems)., , , Coordination compounds are used as catalysts for many industrial processes., Examples include rhodium complex, [(Ph3P)3RhCl], a Wilkinson catalyst, is used for, the hydrogenation of alkenes., , , , Articles can be electroplated with silver and gold much more smoothly and evenly, from solutions of the complexes, [Ag(CN)2]– and [Au(CN)2]– than from a solution of, simple metal ions., , , , In black and white photography, the developed film is fixed by washing with hypo, solution which dissolves the undecomposed AgBr to form a complex ion,, [Ag(S2O3)2]3– ., , , , There is growing interest in the use of chelate therapy in medicinal chemistry. An, example is the treatment of problems caused by the presence of metals in toxic, proportions in plant/animal systems., , , , Thus, excess of copper and iron are removed by the chelating ligands D–, penicillamine and desferrioxime B via the formation of coordination compounds., EDTA is used in the treatment of lead poisoning., , , , Some coordination compounds of platinum effectively inhibit the growth of tumours, / Cancers . Examples are: cis–platin and related compounds., , In Medical Field, , , , , , Cisplatin – It is a Platinum complex & has been found in the treatment of cancer, (tumors), EDTA – It is used for treatment of lead poisoning., British Anti-Lewisite (BAL) – Developed during war time as an anti-dote to an, arsenic containing poisonous gas lewisite., BAL is also used to treat poisoning by As, Hg, Au, Bi, Sb, Pb & Cd., , Tel +91 84330 83279 , 9837603674,
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