CHAPTER - 9 CO-O ORDINATION COMPOUNDS Formulas for coordinationn compounds: Tetraamineaquachloridocobalt (III) chloride ---- [Co(NH 3 ) 4 (H 2 O) Cl]Cl 2 Potassium tetrahydroxozincate (II) ------- K 2 [Zn() 4 ] Potassium trioxalatoaluminate (III) -------- K 3 [A](C 2 O 4 ) 3 ] Dichloridobis (ethane-1, 2 diamine) cobalt (III) -------- [cocl 2 (en) 2 ] + Tetracarbonylnickel(0) --------- [Ni(CO] 4 ] Effective atomic number (EAN) = Z (atomic no.) (O.N.) + 2 (C.N.) [Co (NH 3 ) 5 SO 4 ] Br and [Co (NH 3 ) 5 Br]So 4 are ionization isomers. [Co (NH 3 ) 6 ] [Cr (CN) 6 ] and [Cr (NH 3 ) 6 [Co (CN) 6 ] are co-ordination isomers. [Co (NH 3 ) 5 ONO] 2+ and [Co (NH 3 ) 5 NO 2 ] 2+ are linkage isomers. [Cu (NH 3 ) 2 Cl 2 ] will form geometrical isomers Trans- [Cu (Gly) 2 ] will form optical isomers. Werner theory considers two types of valency in complexes
Primary (O.N.) and Secondary (C.N.) Ligands/ions attached by secondary valency do not ionize. Complex ion. It is an electrically charged radical which is formed by the union of simple cat ion with one or more natural molecules or simple ions. E.g., [Fe(CN) 6 ] 4-, is formed b the union of six CN ions with one Fe 2+ ion. The ions or molecules which surround the central metal atom or ion are called ligands. The non bonded pairs of electrons on the ligands form co-ordinate bonds to the central metal atom or ion by donating these unshared electron pairs into its vacant orbitals. It is because of this reason that the term co-ordination compounds is coined for such substances. Types of complex on the basis of stability. On the basis of stability of complex ion in solution, complex salts are classified into two types: Perfect or penetrating complexes. These are compounds in which complex ion is fairly stable and is either not dissociated or feebly dissociated in solution state. For example, [K 4 [Fe(CN) 6 ] 4K + + [Fe(CN) 6 ] 4- Fe 2 + +6CN - (feebly dissociated) The ferrocyanide ion [Fe(CN) 6 ] 4- is so unsuficiently dissociated that it may be taken as practically undissociated and does not give the test of Fe 2+ or CN - ions. [Co(NH 3 ) 6 ] 3+ is another such complex.
Imperfect or normal complexes. These are compounds in which complex ion is less stable and is reversibly dissociated to give enough simple ions and thus gives their tests. For example, K 2 [Cd(CN) 4 ] 2K + + [Cd(CN) 4 ] 2- Cd 2+ + 4CN - (appreciably dissociated) The IUPAC names of the coordination compounds: [Pt (NH 3 ) 2 Cl(NO 2 )] ------ Diamminechloridonirito -N- platinum(ii) K 3 [Cr(C 2 O 4 ) 3 ] -------- Potassium trioxalatochromate (III) [cocl 2 (en) 2 lcl ---------- Dichloridobis (ethane-1, 2- diamine) cobalt (III) [Co(NH 3 ) 5 (CO 3 )]Cl - ------- Pentaaminecarbonatocobalt (III) chloride Hg[Co(SCN) 4 ] ------- Mercury tetrathathiocyanatocobaltate (III) The valence bond theory (VBT) explains with reasonable success, the formation, magnetic behavior and geometrical shapes of coordination compounds. It, however, fails to provide a quantitative interpretation of magnetic behavior and has nothing to say about the optical properties of these compounds. In nomenclature O.N. of the metal in a complex is written If complex is anion, metal ion ends with ate Ligands (with ite in the end) are written as ending ito, ato (sulphato oxalate etc.) Ligands (with ide in the end) are written as ending o (chloro, bromo) K 4 [Fe (CN) 6 ] is potassiumhexacyanoferrate (II)
The Crystal Field theory (CFT) to coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges), on the degeneracy of d orbital energies of the central metal atom/ion. The splitting of the d orbital provides different electronic arrangements in strong and weak crystal fields. In optical isomers, the designation (+) and (-) or (d) and (l) are put before the complete formula of the complex compound and not before the complex ion showing optical isomerism e.g., (+) K 3 [Cr(C 2 O 4 ) 3 ] and not K 3 (+) [Cr(C 2 O 4 ) 3 ] Ligands are generally capable of donating one or more lone pairs of electrons. Positively charged ligands also donate lone pair of electrons. Therefore are certain ligands which do not have lone pair of electrons but still can act as ligand e.g. ethylene or C 2 H 4 in Zeise s salt is one such ligand. K[PtCl 3 (η 2 C 2 H 4 )] Zeise s salt Oxidation state of Fe in ferrocene is Fe2+. The two cyclopentadienly anion (C5H ) carries one unit negative charge each. Metal carbonyls are not regarded as ogranometallic compounds in the true sence. This is because CO is not an organic compound. However metal carbonyls are studied along with other ogranometallic compounds. KCN contains a metal-carbon bond but it is not considered as an ogranometallic compound.
[Fe (CN) 6 ] 4- is a complex (co-ordination) ion in which Fe 2+ is a metal ion and CN ligand (electron pair donor); ligand co-ordinated to metal ion does not ionize in aqueous solution/medium. Ligand can be unidentate, bidentate, tridentate etc. depending upon the number of electron pairs it donates. NH 3, H 2 O, CO, NO are neutral unidentate CN -, -, Cl - are anionic unidentate (CH 2 NH 2 ) 2, C 2 O 2-4, glycine are bidentate EDTA is hexadentate CN -, NO - 2, etc. can be joined to metal ion of either side (ambidient) Bidentate etc form a ring with metal ion in a complex Chelate, for example nickel dimethyl glyoximate. The co-ordination number (C.N.) of a metal atom in a complex is the total number of bonds the metal atom forms with ligands; (C.N.) = 2 x valency. In complex compounds the central metal atom or ion can act as Lewis acid and ligands as Lewis bases. Transition metals can form complexes in the zero, positive and low negative oxidation states i.e. the centre of co-ordination can be a neutral atom, cation or even an anion in low negative oxidation state.
Chelating ligand is a neutral molecule, free radical or ion (cation or anion) with two or more Ione pairs of electrons on different atoms. Chelates with five membered rings are very stable if these do not contain a double bond e.g. in ethylenediamine. However, Chelates with six membered rings are very stable if the ligands contain double bond e.g. acac. Masking: Masking is the process in which a substance without physical separation of it is so transformed that it does not enter into a particular reaction e.g., masking of Cu 2+ by CN - ion. Application of Co-Ordination Compounds The ability of metal ions to form complexes with a variety of molecular species with different physic-chemical properties is utilized in many ways. For example: In analytical chemistry: Multidentate ligand, EDTA (ethylenediamintetraacetic acid forms highly stable complexes with metal ions like Ca 2+ and Mg 2+. This fact is used to estimate the hardness of water by a simple titration method using standard EDTA solution. A confirmatory test for the detection of copper (II) involves the formation of a deep-blue coloured complex, [Cu(NH 3 ) 4 ] 2+, on addition of ammonia solution to a solution of copper (II) salt. Cu 2+ (aq) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) tetraamminecopper (II) ion (Deep-blue) The separation of Group I precipitate (of qualitative inorganic analysis) of AgCl, Hg 2 Cl 2 and PbCl 2 involves the addition of aqueous ammonia solution to the precipitate, when only silver chlorides dissolve due to the formation of the complex ion, [Ag(NH 3 ) 2 ]+. AgCl(s) + 2NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) + Cl - (aq) Hg 2 Cl 2 and PbCl 2 do not form complex ions with NH 3 and hence, do not dissolve.
A confirmatory test for nickel involves the addition of a solution of dimethylglyoxime, when a scarlet-red coloured precipitate is formed, due to the formation of a chelate complex. Thus: CH 3 C = N 2 + Ni 2+ CH 3 C = N Dimethylglyoxim O H.. O CH 3 C = N N = C CH 3 Ni CH 3 C = N N = C CH 3 O... H O Chelate complex (Scarlet-red) + 2H + Macrocyclic effect: This term refers to the greater thermodynamic stability of a complex with a cyclic polydentate ligands when compared to the complex with a non-cyclic ligands. e.g., Zn(II) complex with ligand; NH NH NH HN is more stable than with NH HN NH 2 H 2 N π- acid ligand : Ligands which are capable of accepting an appreciable amount of π-electron density from the metal atom into empty π or π* orbital of their own are called π-acceptor or π- acid ligands e.g., CO.
ISOMERISM Two different types of isomerism i.e., structural and stereoisomerism are possible for coordination compounds. Structural isomerism. Various types of structural isomerism are: Ionisation isomerism arises when the co=ordination compounds with same molecular formula give different ions in solution e.g., violet [Co(NH 3 ) 5 Br]SO 4 and red [Co(NH 3 ) 5 SO 4 ]Br. Linkage isomerism arises in complexes containing monodentate ligands with more than one donor atoms e.g., NO may be bonded to metal through nitrogen (nitro) or through oxygen (nitrito). Such ligands are also known as ambidentate ligands Co-ordination isomerism involves exchange of ligands between complex cation and complex anion i.e. between co-ordination spheres in a compound. Hydrate isomerism arises in the complexes due to different number of water molecules present in the co-ordination spheres. For example: [Co(H 2 O) 6 ]Cl 3 ; [Co(H 2 O) 5 Cl] Cl 2. H 2 O ; [Co(H 2 O) 4 C: 2 ] Cl. (H 2 O) 2 ; [Co(H 2 O) 3 Cl 3 ] (H 2 O) 3 are hydrate isomers. Co-ordination position isomerism. This type or isomerism is shown by bridged complexes and involve different placement of ligands. it is a special type of coordination isomerism. e.g., (NH 3 ) 4 Co Co(NH 3 ) 2 Cl 2 SO 4 and Cl(NH 3 ) 3 Co Co(NH 3 ) 3 Cl SO 4
Ligand-isomerism: Consider the following two ligands, which are H 2 C CH CH 3 and H 2 C CH 2 CH 2 H 2 N NH 2 NH 2 NH 2 1, 2 diaminopropane (pn) 1, 3 diaminopropane (tn) When these ligand form compounds, they will exhibit ligand isomerism. Example. [Co (pn) 2 Cl 2 ] + and [Co (tn) 2 Cl 2 ] + In extraction of metals: Extraction of metals like silver and gold is carried out by forming their soluble cyanide complexes. For example, 4 Au + 16 CN - + 6H 2 O + O 2 4 [Au(CN) 2 ] - + 12 - dicyanogold (I) ion (Soluble) The solution containing cyanide complex is then treated with zinc when gold is precipited. 2 [Au(CN) 2 ] - + Zn (Zn(CN) 4 ] 2- + 2Au Co-ordination compounds of silver and gold are used as the constituents of electroplating baths for the controlled delivery of Ag + and Au + ions, during electroplating or electro-refining of these metals. The name of a complex compound should not start with a capital letter e.g., [Cu(NH 3 ) 4 ]SO 4 Tetraamminecopper (II) sulphate And not Tetraammine copper (II) sulphate The full name of the complex should be written as one word without any gap.
Formulation of Cobalt(III) chloride-ammonia Complexes Color Formula Solution conductivity Corresponds to Yellow Purple Green Vilote [Co(NH 3 ) 6 ] 3+ 3Cl - [cocl(nh 3 ) 5 ] 2+ 2Cl - [cocl 2 (NH 3 ) 4 ] + Cl - [cocl 2 (NH 3 ) 4 ] + Cl - 1 :3 electrolyte 1 :2 electrolyte 1 :1 electrolyte 1 :1 electrolyte THANK YOU