coordination compound part 1, Cheat Sheet of Chemistry

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rr r Sah. INTRODUCTION Alfred We: ter (1866-1919) recognised that there was no link between the oxidation state of a metal and the number of ligand, bound to it. This allowed him to define the highly stable complex formed between cobalt (III) and six ammonia molecules, arranged symmetrically at the six comers of an octahedron. The key to the puzzle was not the primary valency of the veel ion but the apparently constant number of donor atoms it supported. This ‘magic number’ of six for cobalt (11) was confirmey through a number of experiments, which led to a Nobel Prize for Werner in 1913, He is therefore regarded as the founder of coordination chemistry. In his honour, simple coordination compounds are often called Werner complexes. i Coordination compounds are a very | to living organisms. For instance, chlorop! important class of chemicals. Naturally occurring coordination compounds are vita hyll (the green pigment in plants), haemoglobin (the red pigment of blood), vitamin | 5 i ely. Many enz ini i et By2 are coordination compounds of magnesium, iron and cobalt, respectively. Many enzymes containing a metal ion can be fo Considered as coordination compounds. For example, carboxypeptidase, a hydrolytic enzyme, important in digestion, contains a zinc jon coordinated to several amino acid residues of the protein, Coordination compounds are widely used in medical diagnosis and therapy, Many catalysts that are commonly used in the chemical industry are coordination compounds, te molecular or higher order_compounds are further @ CLASSIFICATION OF CHEMICAL COMPOUNDS classified into two categories—@ Double salts — and . © Complex salts or coordination compounds. Chemical compounds can be classified in the following ways: a a al] x 19.1.1 [ee sta ne nn ve Doublesat ie | 1 Za Chemical Compounds ‘ @ A mixwre of ferrous sulphate (FeSO,) | and] ¢ wv Simple or First Addition or Molecular or ammonium sulphate [(NH,),SO,] in the ratio of their AS ‘ od 4294 < Srcuns ihe Orde Conromee formula masses, when dissolved in water, concentrated and | f : cooled, forms Hght green crystals of ferrous ammonium| / Double Salt a gpsceteatebay- vere sulphate hexahydrate [FeSO ,-(NIL,),$O ,-GH0] commonk known as Mohr salt. a mixture of potassium <* . Simple or first order compounds sulphate (K,S0,) and aluminium sulphate [Al,(S0,),] in the ratio of their formula masses, when dissolved in water and Atoms of two or more Glements)combine together withthe | crystallised, forms colourless crystals of potash help of electrovalency or covalency to form simple or first order alum [K,$0 pA1,(S0,),-2411,0]. These salts are stable in the solid state, As the constituent ions are arranged in definite order al compounds. . Examples: Sodium chloride (NaCl, ionic or electrovalent | well-defined positions of the crystal lattice, their composition compound), hydrogen chloride (HCL, covalent compound). | remains unaltered in the solid state. But they dissociate into c ea ais of molecular compounds their respective fons when dissolved in water. These salts are 7 called double salts. Two_or more ( imple compounds) combine to form a ——— iti . mpounds are A A molecular or an addition compound. These compo | Definition 4t two or more simple salts, mixed in their| relatively stable in the solid state but some of them do not sto iometric proportions, combine to form a new salt) retain their identity in aqueous solutions. The formation of “| which retains its identity in solid state but completely, dissociates in aqueous solution to Rive its constituent ions | the salt formed is called a double salt. | molecular compounds cannot be explained with the help of | electrovalency and covalency. Example: Various hydrated salts (CuSO,-5H,0; ZnSO,-7H,0), ammoniated salts (CuSO,-4NH,; Example: When camallite is dissolved in water, it dissociates 1 CoCl,-6NH3), ferrocyanide salts [Fe(CN),-4KCN], *|form_ K*, Mg*t and Cl ions, Similarly, Mohr_s2 alums [K,SO,-Al,(SO,),-24H,0], etc. € dissociates into Fe?*, NH} and so; ions. a 0) KCI-MgCl,-611,0 = K* + Mg? + 3C1- + 6H,0 Camallite 4 Coordination Compounds or Complex Compounds k shows the presence of Kt ion and a complex ferrocyanide [Fe(CN),]4- ion, which gives a prussian blue precipitate eS04(NII,),S04-6H,O = Fe?* + 2NH} +2802" +6H,0 Mohr salt AA4 (3 The following experiment® ascertain the presence of e?*,NHj and S02- ions in the aqueous solution of Mohr alt. Divide the aqueous solution of Mohr salt into three parts: ) First Part: On adding a small amount of H. 250, and a few rops of Potassium ferricyanide solution to it, a blue. recipitate (Turnbull’s blue) is. formed. This shows the resence of Fe2* ions in the solution. @ © Second part: On oiling the solution with a few crystals of solid NaOH and assing the evolved gas through Nessler’s reagent, a brown recipitate is formed, indicating the presence of NH ions in he solution. @ Third part: On adding a small amount of HCI with FeCl, solution. When electricity is passed through this solution, the ferrocyanide ion moves towards the positive electrode indicating ‘the absence of free Fe?* ions in the compound. Therefore, in the reaction between Fe(CN), and KCN, Fee and CN7 complex ion, [Fe(CN),]‘-, which retains its identity in aqueous solution, a4 (ay) ions combine and form a new stable Fe(CN), + 4KCN == K,[Fe(CN)g] Ferrous cyanide Potassiumcyanide potassium ferrocyanide (Complex compound) K,[Fe(CN),] —= 4K* + [Fe(CN)g]*~ (Complex ion) Coordination compounds are commonly called ind BaCl, solution to it, a white precipitate is formed, which complexes. Ae s insoluble in acid, indicating the presence of $0, ions in @5 he solution. When two or more simple molecules or ions | A nccammerememneneen v ae combine to produce a well-organised and aggregated AK double salt in aqueous solution dissociates completely molecule (or ion) which retains its identity in solid as well into its constituent ions. The parent compound can be as in dissolved states, it is called a complex compound paneer on Meas the aqueous solution. 4 (or ion). Class} cat ‘a of double salts: Double salts are generally lation of some complexes classified into three categories: 494 84 (5) Formation and, dissoci - P Formation Dissociation (in water) Class General formula Example(s) “2 a lt om Potash alum: yx "| |CuSO,+4NH, == [Cu(NH3),]50, = , M,S04°M)(S0,),:24H,0 K,$0,-Aly(S0,),-2411,0 xy [Cu(NHy),180, [Cu(NH),] *+802- where, M(I) = Na*, K*,/Chrome alum: a et d Dafel, + 2KI —= Ky[Hgl 4) | K,[figt,] —= 2K" + [Hg], 2- Alum NH} etc, and M(II) =|K2S04-Cr,(S0,),-24H,0 Stet, 2bh'6's? |KoL Eel 4) [Hg 4 gst At, Rett, cra, |Perric alum: Ptcl, +2KCl— K,[PtCly] == 2K* + [PtCly]?~ 2 : K,S04-Feg(SO,4),-24H,0 K,[PtCl,] RS Co etc. a i. 4 — Mohr salt: ~~ AgNO, +2NH, == [Ae(NH,),1NO, <— Schonite |MSO,-M,S0,-6H,O FeSO,-(NI14),S0,-611,0 . [Ag(NH3),]NO, [Ag(NH),]* + NOZ I ou Natural carnallite: 4¢ 7 ; MEMClySHg0 KChMgC1,-6H0 Difference between double salt and compiex salt zd s . Topic Double salt Complex sale ; tpg autor mprencentconabvarspers Preedtpaao -* TEES complex compounds or coordination compounds Formed by the com-|Furmed by the combi. CS . . K bination of two or|nation of two or more A mixture of ferrous cyanide [Fe(CN),] and potassium | Sirticture more simple salts. [simple salts or by any cyanide (KCN), in 1:4 mole ratio, when dissolved in water /other method. and crystallised, forms light yellow crystals with a chemical — [4¥ Exists only in solid|Exists both in_solid as composition of Fe(Cn)s- KEN. The aqueous solution of this Existence | state. well as in dissolved salt does not indicate the presence of Fe2+ or CN7 ion, but state. Qa-5_ Q-2 B58 way ar a hae Coordination Compounds or Complex Compounds 793 (Ligands (satisfy six secondary valencies) @ In [Co(NH,),]Cl,, the primary valency of Co3* ion is 3 and is satisfied | satisfy three primary valencies [cota Je —y— Coordination number (= 6)| € ‘s é| 3 & £ e 3 by 3 chloride ions. In aqueous solution, the 3 chloride jons get separated from the central metal ion and move around freely in the solution. The sec- ondary valency of Co3* ion is 6 and is satisfied by 6 NH, molecules. In aqueous solution, Co#+ ion along with the bonded NH, molecules remain together forming the coordination sphere. Cy Anionic ligands (satisfy 2 primary and 2 secondary valencies) Central metal ion Qin [CoCl,(NH,),]CI, the primary valency of Co* ion is 3 and is satis- fied by 1 C1~ sphere. In aqueous solution only'f Cl- ion gets separated from the central metal ion and moves around freely in the solution. The remaining 2 Cl ions do not get separated from the central metal (Co3*) ion as they are used to satisfy two secondary valencies of Co3+ ion. That means CI- Coordination number (=4+2=6) Coordination sphere ions simultaneously satisfy both the primary and the secondary valencies. Since the 2 Cl~ ions are bonded to the Co3* ion by coordinate bonds and are used to satisfy secondary valencies, they do not get separated in aqueous solution and hence, cannot move freely in the solution. Central Ligands (satisfy 4 metal secondary valencies) atom [ (CO), Coordination Coordination sphere! number (=4) @ In Ni(CO),, there is no net charge on (Ni) and hence, only the second- —— — ary valencies of the central metal ion are satisfied by the 4 CO molecules (neutral ligands), Anionic ligands (satisfy 2 primary and 2 secondary valencies) Central | Neutral ligands metal (satisfy 2 secondary fon [@eiany, “ valencies) (Coordination number] (=2+2=4) Coordination sphere] Oln [PtCl,(NH3),], Pt2+ ion has two primary valencies, which are satis- fied by two Cl~ ions lying inside the coordination sphere. Further, Pt2+ has four secondary valencies, which are satisfied by two Cl- ions and two NH, molecules. As the Cl~ ions are used in satisfying the secondary valencies, they do not get separated from the Pt?* ion in aqueous solution. Hence, the coordination sphere of this complex remains electrically neutral and there are no ionisable species in this complex. Difference between Primary valency and Secondary valency Primary Valency Secondary Valency his valency is normally ionsiable. KEE This valency is non-ionsiable. hese valencies are satisfied by negatively charged ions. $< {It is commonly satisfied by neutral and negatively charged, eH sometimes Positively ‘Charged ligands. is equal to the positive charge on central metal ion/atom. ++ eee ee This valency equals the number of ligand’s atoms coordinated to the metal. It is also called coordination number of the metal. ion lying outside and 2 Cl~ ions lying inside the coordination} planation of the bahaviour of cobalt (Ill) chloride- prepared by Tassaert. These compounds had different colours and other properties which are shown in the following table, nmonia complexes on the basis of Werner's theory Werner_prepared several cobalt (III) chloride-ammonia The results of the conductivity measurement and reaction with mplexes in addition to the compound (CoCl,-6NH,) AgNO, give us some idea about the number of ionisable chloride ions present in each complex. << fc AME (794 FAR _ chhaya Chemistry (x1) Pari-1 *K ee RS | | No. of ions in | No. of moles | solution of AgCl_ ‘Compound | | Composition | Colour | (obtained | precipitated | | | fram molar | per mole of een Bes _|conduetivity) | the complex | A |Co Cl, NH Orange 4 3 |B SoCl,-5NH3) Pink 3 2 = he | fo} CoCl,4NHy, Green 2 1 | D | CoCl,-4NH5| Violet 2 1 Explanation: The above experimental results can be explained if it is assumed that @) Co3* ion is banded to six groups (either NH, or Cl~ or both) by primary valency, and @ the compounds are formulated as shown below, where the molecules or ions within the square brackets form a single entity that does not dissociate even in the dissolved state, while only the ions lying outside the brackets undergo dissociation. Hence, they react with AgNO, to give a precipitate of AgCl. | H,0 ., | Coct,-6NH, = [Co(NH,),1Cl, == < + CA] Alornse) Co(NH,)_J3+ + 3C17 2485 gagci. G rane [Co(NH,), ae {3 mol) ions — ~~ 1,0 PCoCI,-SNH, = [CoCl(NH,),]Cl, == jon gn [CoCl(NH,),]** +2CI- 248 gage ss SEES ean 3ions (mol) ay 1,0 | Fe [cols 4NH, = [GoCly(NH,) JC = Elaseen Age [CoCl,(NHg),]*+ Cl” > AgClt —_—— (1 mol) 2ions H,0 CoCly-4NHy = [CoCl,(NH3) ,]Cl = Bw iolet) as aN [CoCl,(NH,),}* + Cl- AB’ age ge? (1 mol) ony ONS 2ions — oO. | As all the Cl~ ions of complex A are present outside the coordination sphere, they react with Agt ions to_form_a precipitate of AgCl (3 mol). For complexes B, C and D, two, one and one Cl~ ions are present outside the coordination sphere. ccordingly 2. mol, | mol and 1 mol of AgCl get precipitated. ans (45 ) It was later observed that complexes C and D are geometric; isomers of each other. i: cl i H,N NH, cl Ne aN an | Sik wr ‘Syn NH, Cl (a) 0) Primary valency Secondary valency Werner's representation of the complexes: (a) [Co(NIf,), Gly (b) [GoCl,(NHy) 161 > WARM UP EXERCISE What are first order and higher order compounds, Give examples. 2. Write four differences hetween double salts and compl salts, (3. How will you identify the ions present in Mohr salt? KA. How will you prove that [Fe(CN),-4KCN] is a complex salt we. Classify double salts with examples. . What is primary valency? Why is it known as ionisable valency? * as What is secondary valency? Why is it known as none 4” jonisable valency? 8. Determine the coordination number of Ni and Co in ihe 4% complexes [Ni(CO),] and | (CONN), Br. IMPORTANT TERMS REGARDING COORDINATION COMPOUNDS Coordination entity (Ast co- ovdimakion 4 ee Weve Definition ‘charged lon neutral species, capable of retaining its identity in the “soit It is defined as an electricull or dissolved-state and con: ing of a central metal atom (ot| ft/ion) surrounded by a group of coordinately bonded ions of fl «NH triethylenetetramine $e AS) HN-CHach, (rien) . Pentadentate ligands with 8 CH, coo | Pentadentate ligands | neutral donor and anionic s QOCCH,—NH— CH, —CH,- Non, coo" groups ethylenediaminetriacetato (eatas-)" | gaa) ser Haws) Ke 0 RE. tl | a Hexadentate ligands with se fO—-C—Hy Ry CH,—CH, — H,—C—08 I Hexadentate ligands ppeutral donor and anionic So- —c— HOS 2 MoH, —c—04 | groups ; § . _ethylenediaminetetraacetato (edta*-) ; aT > = i mbident or ambidentate ligand oa [* Monodentate ambident Bideftate ambident. | IMO” A monodentate ligand \ which contains more ligand ligand | ° 4 than one donor atom but only one of them is used at a time et@=N: or 1C=Ni> to form a coordinate bond with the central metal atom is cyanide ion 5 ° c led an ambident ligand. j @ Sy OK SO 8 — - - <5—C=N: or:8—CEN: if ‘e* Since there are known examples 0 of bidentate ambident Siphocyanie “a s? og Ay g@nds, the above definition can be modified as follows: dithiooxalate ion { ss A ligand which contains more than one type of donor oat nitrite ion aCe. oms but only one type of atom (or atoms) is/are used at a — ne to form coordinate bond (or bonds) with the central metal om is called an.ambident ligand. Arr ample: @ Nitrite ion (NOZ) is an ambident ligand because it.can coordinate either through N_ or through O to a central metal atom (or ion). @ Similarly, SCN™ ion is an ambident ligand because it can coordinate through ’ -s Ambident ligands are capable of forming Unkage cy isomers, e.g, [Co(NH,).(OND)ICI & [Co(NH,).(NO,)]CI. Flexidentate ligand $3. Lem sulphur or nitrogen atom. @ Pithiooxalate fon (a bidentate ligand) can form two coordinate bonds at a time with the central metal atom, either through two O -atoms or through two S-atoms, Hence, it is a bidentate amblident ligand, al O5508 Behave Example; sg-ON [Hehavesasa Ox (Behaves as a_ ple; |@ of Soe bidentareTigand) gan Oe and} — (800 AN) chhaya Chemistry (xt) Port-1 \s m @ [Co(NH,),CO,]Br Here carbonate ion behaves as a bidentate ligand. It satisfies 2 primary and 2 valencies. ur [Co(NH3),CO,]Br Here carbonate ion behaves as a__monodentate It L ligand. satisfies 2 primary and secondary valencies. secondary are ® Tridentate (S, N, O) cysteine [HS—CH,—CH(NH,)— COOH] ha behaves as a bidentate ligand in 3 |} ways (S, N; N, O; S, O) as shown " “ below. Hence, cysteine is a — flexidentate ligand. | ne i \- | oO | | cH sp 2 | | jus "CH “e asi | | is 7 ie iy NH, Pa © Many polydentate ligand behave as flexidentate ligands. For example— EDTA is primarily a hexadentate ligand. However, in certain cases it behaves as a w € “Se? i. onenen ligands and central metal atom | uchbtxample: F, ¢ F-, Cl”, OH~, NH, H,0, CO, CN", etc. Example: Bridged complexes may be binuclear, trinuclear, ey, Examples of binuulear complexes are— wh bridging ligand bridging ligand \ H. ay cl cl cl Nw NLZO™N NN ZN ZN, (en),Co Co(en), a - No bridging ligand | _~ | H bridging ligand [ slassitication of ligands s based on the nature of bonds ! “hy Classical ligands: They form coordinate bonds with the| <-Central metal atom or ion by donation of an unshared pair of eJectrons, - _1t) Non-classical ligands: They form coordinate bonds wit & Sthe central metal atom or ion by using the electron pai of the x -bond. | _ Example: C,Hy, Collg, C,I5 (cyclopentadienyl anion) | 3~ SL oca «x pentadentate or even a tetradentate ligand. FevVetait bq -234 Ifa bidentate or a polydentate ligand forms two | or more coordinate bonds simultaneously with the same central metal atom (or ion) through two or more donor atoms forming one or more ring systems in the coordination sphere, then that ligand is called a chelating ligand and the 1S Chelating ligand complex thus formed is called a chelate. Example: en, gly”, ox? 2-, dmg~, acac7, bipy, dien, etc. . ws | Two , molecules of bidentate Tigand ethylenediamine, with two neutral donor atoms, are coordinated to the Cu?* jon and the coordination [Cu(en),]2* of a chelate. Hence, ethylenediamine (H,N—CH,CH, See article no. 9.11.3 for examples of chelates. Bridging ligand forms sphere —NH,) is a chelating ligand. | Definition er atom of some of the monodentate ligands ay have more than one lone pairs of electrons and hence it may co-ordinate simultaneously with two or more metal atoms. Such a ligand acts as a bridge between the two metal atoms and is known as bridging ligand. The resulting complex is known as polynuclear complex or bridged complex. ey &, O\ oo Fe Ferrocene [Fe" (13-Cslis)2] | aa Zeise’s salt K[Pt Cl,(17-C,H,)] In these complexes, the number following 7(eta) | represents the number of C -atoms bound to the metal. x -acid ligand : These ligands, on one hand, form coordinat {bonds with the central metal atom by donating a pair 9] electrons. On the other hand, they form another bond (11 bond) with the central metal atom by accepting @ “ K < electron pair from the metal and the process is known 4 back-bonding. e.g., CO, PR, etc. a Exampl To explain the stability of coordination compounds, Sidgwidl put forward a rule on the basis of effective atomic number °| the central metal atom. In coordination compounds, effectit" atomic number of the central metal. atom is equal to It atomic number of the noble gas which follows the cent Mac :: —> Me Ni+4CO—+[Ni(CO),} c=6: Bio vead 3.839 SIDGWICK THEORY: EFFECTIVE ATOMIC NUMBER RULE 12% +c) ~Woke. ‘metal atom in the periodic table.