Chemistry Chapter 5 - Class XII

Chemistry 118 In the previous Unit we learnt that the transition metals form a large number of complex compounds in which the metal atoms are bound to a number of anions or neutral molecules by sharing of electrons. In modern terminology such compounds are called coordination compounds. 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 these compounds as 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. Coordination Compounds After studying this Unit, you will be able to • appreciate the postulates of Werner’s theory of coordination compounds; • know the meaning of the terms: coordination entity, central atom/ ion, ligand, coordination number, coordination sphere, coordination polyhedron, oxidation number, homoleptic and heteroleptic; • learn the rules of nomenclature of coordination compounds; • write the formulas and names of mononuclear coordination compounds; • define different types of isomerism in coordination compounds; • understand the nature of bonding in coordination compounds in terms of the Valence Bond and Crystal Field theories; • appreciate the importance and applications of coordination compounds in our day to day life. Objectives Coordination Compounds are the backbone of modern inorganic and bio–inorganic chemistry and chemical industry. Coordination Compounds Alfred Werner (1866-1919), a Swiss chemist was the first to formulate his ideas about the structures of coordination compounds. He prepared and characterised a large number of coordination compounds and studied their physical and chemical behaviour by simple experimental techniques. Werner proposed the concept of a primary valence and a secondary valence for a metal ion. Binary compounds such as CrCl3, CoCl2 or PdCl2 have primary valence of 3, 2 and 2 respectively. In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution. 5.1 Werner’s Theory ofTheory of Theory of Coordination CompoundsCompounds Compounds 5 Unit Reprint 2026-27119 Coordination Compounds 1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl 1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl 1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl 1 mol CoCl3.4NH3 (Violet) gave 1 mol AgCl These observations, together with the results of conductivity measurements in solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 5.1, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions. Werner proposed the term secondary valence for the number of groups bound directly to the metal ion; in each of these examples the secondary valences are six. Note that the last two compounds in Table 5.1 have identical empirical formula, CoCl3.4NH3, but distinct properties. Such compounds are termed as isomers. Werner in 1898, propounded his theory of coordination compounds. The main postulates are: 1. In coordination compounds metals show two types of linkages (valences)-primary and secondary. 2. The primary valences are normally ionisable and are satisfied by negative ions. 3. The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. 4. The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. In modern formulations, such spatial arrangements are called coordination polyhedra. The species within the square bracket are coordination entities or complexes and the ions outside the square bracket are called counter ions. He further postulated that octahedral, tetrahedral and square planar geometrical shapes are more common in coordination compounds of transition metals. Thus, [Co(NH3 )6 ] 3+, [CoCl(NH3 )5 ] 2+ and [CoCl2 (NH3 )4 ] + are octahedral entities, while [Ni(CO)4 ] and [PtCl4 ] 2– are tetrahedral and square planar, respectively. Colour Formula Solution conductivity corresponds to Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes Yellow [Co(NH3)6] 3+3Cl– 1:3 electrolyte Purple [CoCl(NH3)5] 2+2Cl– 1:2 electrolyte Green [CoCl2(NH3)4] +Cl– 1:1 electrolyte Violet [CoCl2(NH3)4] +Cl– 1:1 electrolyte Reprint 2026-27Chemistry 120 (i) Secondary 4 (ii) Secondary 6 (iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4 On the basis of the following observations made with aqueous solutions, assign secondary valences to metals in the following compounds: Solution SolutionSolution Difference between a double salt and a complex Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, KCl.MgCl2.6H2O, Mohr’s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc. dissociate into simple ions completely when dissolved in water. However, complex ions such as [Fe(CN)6] 4– of K4 [Fe(CN)6] do not dissociate into Fe2+ and CN– ions. Formula Moles of AgCl precipitated per mole of the compounds with excess AgNO3 (i) PdCl2 .4NH3 2 (ii) NiCl2 .6H2O 2 (iii) PtCl4 .2HCl 0 (iv) CoCl3 .4NH3 1 (v) PtCl2 .2NH3 0 Example 5.1 Example 5.1Example 5.1 Werner was born on December 12, 1866, in Mülhouse, a small community in the French province of Alsace. His study of chemistry began in Karlsruhe (Germany) and continued in Zurich (Switzerland), where in his doctoral thesis in 1890, he explained the difference in properties of certain nitrogen containing organic substances on the basis of isomerism. He extended vant Hoff’s theory of tetrahedral carbon atom and modified it for nitrogen. Werner showed optical and electrical differences between complex compounds based on physical measurements. In fact, Werner was the first to discover optical activity in certain coordination compounds. He, at the age of 29 years became a full professor at Technische Hochschule in Zurich in 1895. Alfred Werner was a chemist and educationist. His accomplishments included the development of the theory of coordination compounds. This theory, in which Werner proposed revolutionary ideas about how atoms and molecules are linked together, was formulated in a span of only three years, from 1890 to 1893. The remainder of his career was spent gathering the experimental support required to validate his new ideas. Werner became the first Swiss chemist to win the Nobel Prize in 1913 for his work on the linkage of atoms and the coordination theory. (1866-1919) Reprint 2026-27121 Coordination Compounds (a) 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+ . (b) 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. (c) Ligands The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as Cl– , small molecules such as H2O or NH3 , larger molecules such as H2NCH2CH2NH2 or N(CH2CH2NH2 )3 or even macromolecules, such as proteins. When a ligand is bound to a metal ion through a single donor atom, as with Cl– , H2O or NH3 , the ligand is said to be unidentate. When a ligand can bind through two donor atoms as in H2NCH2CH2NH2 (ethane-1,2-diamine) or C2O4 2– (oxalate), the ligand is said to be didentate and when several donor atoms are present in a single ligand as in N(CH2CH2NH2 )3 , the ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion. When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. Such complexes, called chelate complexes tend to be more stable than similar complexes containing unidentate ligands. Ligand which has two different donor atoms and either of the two ligetes in the complex is called ambidentate ligand. Examples of such ligands are the NO2 – and SCN– ions. NO2 – ion can coordinate either through nitrogen or through oxygen to a central metal atom/ion. Similarly, SCN– ion can coordinate through the sulphur or nitrogen atom. (d) Coordination number The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. 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 C2O4 2– and en (ethane-1,2-diamine) are didentate ligands. 5.2 Definitions of Some Important ImportantImportant Terms Pertaining to Coordination Compounds CompoundsCompounds Reprint 2026-27Chemistry 122 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. (e) 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. The ionisable groups are written outside the bracket and are called counter ions. For example, in the complex K4 [Fe(CN)6 ], the coordination sphere is [Fe(CN)6 ] 4– and the counter ion is K+ . (f) Coordination polyhedron The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral. For example, [Co(NH3 )6 ] 3+ is octahedral, [Ni(CO)4 ] is tetrahedral and [PtCl4 ] 2– is square planar. Fig. 5.1 shows the shapes of different coordination polyhedra. 5.3 Nomenclature of Coordination CompoundsCompounds Compounds (g) Oxidation number of central atom 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). (h) Homoleptic and heteroleptic complexes Complexes in which a metal is bound to only one kind of donor groups, e.g., [Co(NH3)6] 3+, are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH3 )4Cl2 ] + , are known as heteroleptic. Nomenclature is important in Coordination Chemistry because of the need to have an unambiguous method of describing formulas and writing systematic names, particularly when dealing with isomers. The formulas and names adopted for coordination entities are based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC). Fig. 5.1: Shapes of different coordination polyhedra. M represents the central atom/ion and L, a unidentate ligand. Reprint 2026-27123 Coordination Compounds The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom. The following rules are applied while writing the formulas: (i) The central atom is listed first. (ii) The ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge. (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. The following rules are used when naming coordination compounds: (i) The cation is named first in both positively and negatively charged coordination entities. (ii) The ligands are named in an alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula). (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. While writing the formula of coordination entity, these are enclosed in brackets ( ). (iv) Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses. For example, [NiCl2 (PPh3 )2 ] is named as dichloridobis(triphenylphosphine)nickel(II). (v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis. (vi) 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. If the complex ion is an anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion,   2 Co SCN 4      is called cobaltate. For some metals, the Latin names are used in the complex anions, e.g., ferrate for Fe. 5.3.2 Naming of Mononuclear Coordination Compounds Note: The 2004 IUPAC draft recommends that ligands will be sorted alphabetically, irrespective of charge. Note: The 2004 IUPAC draft recommends that anionic ligands will end with–ido so that chloro would become chlorido, etc. 5.3.1 Formulas of Mononuclear Coordination Entities Reprint 2026-27Chemistry 124 (vii) The neutral complex molecule is named similar to that of the complex cation. The following examples illustrate the nomenclature for coordination compounds. 1. [Cr(NH3)3(H2O)3]Cl3 is named as: triamminetriaquachromium(III) chloride Explanation: The complex ion is inside the square bracket, which is a cation. The amine ligands are named before the aqua ligands according to alphabetical order. Since there are three chloride ions in the compound, the charge on the complex ion must be +3 (since the compound is electrically neutral). From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number of the metal. In this example, all the ligands are neutral molecules. Therefore, the oxidation number of chromium must be the same as the charge of the complex ion, +3. 2. [Co(H2NCH2CH2NH2)3]2(SO4)3 is named as: tris(ethane-1,2–diamine)cobalt(III) sulphate Explanation: The sulphate is the counter anion in this molecule. Since it takes 3 sulphates to bond with two complex cations, the charge on each complex cation must be +3. Further, ethane-1,2– diamine is a neutral molecule, so the oxidation number of cobalt in the complex ion must be +3. Remember that you never have to indicate the number of cations and anions in the name of an ionic compound. 3. [Ag(NH3)2][Ag(CN)2] is named as: diamminesilver(I)dicyanidoargentate(I) 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) (a) [Co(NH3 ) 4 (H2O)Cl]Cl 2 (b) K2 [Zn(OH)4 ] (c) K3 [Al(C2O4 ) 3 ] (d) [CoCl 2 (en)2 ] + (e) [Ni(CO)4 ] Write the IUPAC names of the following coordination compounds: (a) [Pt(NH3)2Cl(NO2)] (b) K3[Cr(C2O4)3] (c) [CoCl2(en)2]Cl (d) [Co(NH3)5(CO3)]Cl (e) Hg[Co(SCN)4] (a) diamminechloridonitrito-N-platinum(II) (b) potassium trioxalatochromate(III) (c) dichloridobis(ethane-1,2-diamine)cobalt(III) chloride (d) pentaamminecarbonatocobalt(III) chloride (e) mercury (I) tetrathiocyanato-S-cobaltate(III) Example 5.2 Example 5.2Example 5.2 Solution SolutionSolution Example 5.3 Example 5.3Example 5.3 Solution SolutionSolution Notice how the name of the metal differs in cation and anion even though they contain the same metal ions. Reprint 2026-27125 Coordination Compounds Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms. Because of the different arrangement of atoms, they differ in one or more physical or chemical properties. Two principal types of isomerism are known among coordination compounds. Each of which can be further subdivided. (a) Stereoisomerism (i) Geometrical isomerism (ii) Optical isomerism (b) Structural isomerism (i) Linkage isomerism (ii) Coordination isomerism (iii) Ionisation isomerism (iv) Solvate isomerism Stereoisomers have the same chemical formula and chemical bonds but they have different spatial arrangement. Structural isomers have different bonds. A detailed account of these isomers are given below. This type of isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found with coordination numbers 4 and 6. In a square planar complex of formula [MX2L2] (X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer as depicted in Fig. 5.2. Other square planar complex of the type MABXL (where A, B, X, L are unidentates) shows three isomers-two cis and one trans. You may attempt to draw these structures. Such isomerism is not possible for a tetrahedral geometry but similar behaviour is possible in octahedral complexes of formula [MX2L4] in which the two ligands X may be oriented cis or trans to each other (Fig. 5.3). 5.4 Isomerism in Isomerism inIsomerism in Coordination Compounds CompoundsCompounds Intext Questions 5.1 Write the formulas for the following coordination compounds: (i) tetraamminediaquacobalt(III) chloride (ii) potassium tetracyanidonickelate(II) (iii) tris(ethane–1,2–diamine) chromium(III) chloride (iv) amminebromidochloridonitrito-N-platinate(II) (v) dichloridobis(ethane–1,2–diamine)platinum(IV) nitrate (vi) iron(III) hexacyanidoferrate(II) 5.2 Write the IUPAC names of the following coordination compounds: (i) [Co(NH3)6]Cl3 (ii) [Co(NH3)5Cl]Cl2 (iii) K3[Fe(CN)6] (iv) K3[Fe(C2O4)3] (v) K2[PdCl4] (vi) [Pt(NH3)2Cl(NH2CH3)]Cl 5.4.1 Geometric Isomerism Fig. 5.2: Geometrical isomers (cis and trans) of Pt [NH3 ) 2Cl2 ] Co Cl Cl N H3 N H3 N H3 N H3 + Co Cl Cl N H3 N H3 N H3 N H3 + cis trans Fig. 5.3: Geometrical isomers (cis and trans) of [Co(NH3 ) 4Cl2 ] + Reprint 2026-27Chemistry 126 This type of isomerism also arises when didentate ligands L – L [e.g., NH2 CH2 CH2 NH2 (en)] are present in complexes of formula [MX2 (L– L)2 ] (Fig. 5.4). Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma3b3] like [Co(NH3)3(NO2)3]. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer (Fig. 5.5). Fig. 5.4: Geometrical isomers (cis and trans) of [CoCl2 (en)2 ] Why is geometrical isomerism not possible in tetrahedral complexes having two different types of unidentate ligands coordinated with the central metal ion ? Tetrahedral complexes do not show geometrical isomerism because the relative positions of the unidentate ligands attached to the central metal atom are the same with respect to each other. Solution SolutionSolution Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left). Optical isomerism is common in octahedral complexes involving didentate ligands (Fig. 5.6). In a coordination entity of the type [PtCl2(en)2] 2+, only the cis-isomer shows optical activity (Fig. 5.7). 5.4.2 Optical Isomerism Fig.5.6: Optical isomers (d and l) of [Co(en)3 ] 3+ Fig.5.7 Optical isomers (d and l) of cis- [PtCl2 (en)2 ] 2+ Fig. 5.5 The facial (fac) and meridional (mer) isomers of [Co(NH3 ) 3 (NO2 ) 3 ] Example 5.4 Reprint 2026-27127 Coordination Compounds Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS– , which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. Jørgensen discovered such behaviour in the complex [Co(NH3 )5 (NO2 )]Cl2 , which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO2 ). This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH3 )6 ][Cr(CN)6 ], in which the NH3 ligands are bound to Co3+ and the CN– ligands to Cr3+. In its coordination isomer [Cr(NH3 )6 ][Co(CN)6 ], the NH3 ligands are bound to Cr3+ and the CN– ligands to Co3+ . This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3 )5 (SO4 )]Br and [Co(NH3 )5Br]SO4 . 5.4.3 Linkage Isomerism 5.4.4 Coordination Isomerism 5.4.5 Ionisation Isomerism Out of the following two coordination entities which is chiral (optically active)? (a) cis-[CrCl2 (ox)2 ] 3– (b) trans-[CrCl2 (ox)2 ] 3– The two entities are represented as Draw structures of geometrical isomers of [Fe(NH3 )2 (CN)4 ] – Solution SolutionSolution Out of the two, (a) cis - [CrCl2 (ox)2 ] 3- is chiral (optically active). Example 5.5 Example 5.5Example 5.5 Solution SolutionSolution Example 5.6 Reprint 2026-27