Pharmacy - Complex titrations - v8 (2010).ppt ...

1 Forming reactionsComplex forming reactionsComplex forming reactionsComplex forming reactionsand complexometryand complexometryG bor Galb csG bor Galb csMixing of salt solutions usually does not result in any observablechanges. The properties of the species originally present persistand no new properties can be detected ( no precipitate, nocolor change, etc.). Examples: Complex forming reactionsComplex forming reactionsBut it is not always the see, for instance:FeCl3+ KF ?yellow colorless colorless NaCl + KNO3 or ZnSO4+ MgCl2 CoCl2+ KSCN ?

2 Pink colorless blueplus, metal ions are also hidden from their usual these cases, the reaction between the ions resulted in theformation ofcomplex ions(compounds): Complex forming reactionsComplex forming reactionsFe3++ 6 F-=[FeF6]3-Co2++ SCN-= [Co(SCN)]+ Complex compounds contain acentral metal ionsurrounded byother ion(s) or molecule(s) neutral molecules ( ammonia, water).The driving force behind the formation of Complex ions is thatcertain atoms in the ligands donate one of their free electronpairs to the central metal ion (donor atoms), to fill its valenceshell.

3 This is essentially an acid-base reaction according to bond type is calledcoordinative number of electron pairs a metal ion is capable of receivingdepends on it radius and electron configuration ( the M shellof Si4+can accept a total of 18 electrons) and also the structureof the ligand. This number is called thecoordination forming reactionsComplex forming reactionsIf there is only one donor atom in the ligand, it is calledmonodentate, if two then it isbidentate, :coordination number: 6monodentate ligandscoordination number: 4bidentate there is more than one type of ligand present in a Complex , itis called amixed :[Fe(H2O)4(SCN)2]+ Complex forming reactionsComplex forming reactionsIn some complexes, there may be even more than one centralmetal ion.

4 These aremulti center :Mn2(CO)10Fe2(CO)9 The formula of a Complex ion is put in square brackets, and it isplaced at the front if its charge is positive, or at the back ifnegative. Examples:[C (NH)]ClK[F (CN)]Nomenclature of Complex compoundsNomenclature of Complex compounds[Co(NH3)6]Cl3K3[Fe(CN)6]Thename ofcomplex cationsstarts with the greek form of thecoordination number (mono-, di-, tri-, etc.), and then comes thename of the ligand (an o tag is also appended to it if the liganditself has charge), plus the name of the metal ion with its valencestate indicated.

5 Examples:[Ag(NH3)2]+diammine-silver (I) ion[Ni(H2O)6]2+hexaaqua-nickel (II) ion[CuCl4]2-tetrachloro-copper (II) ofcomplex anionsis similar, but with an -ate endingadded to the name of the metal ion. Examples:[BiI4]-tetraiodo-bismutate (III) ionNomenclature of Complex compoundsNomenclature of Complex compounds[Al(OH)4]-tetrahydroxo-aluminat e (III) ion[FeF6]-hexafluoro-ferrate (III) ionInthecaseofaneutralcomplex:[HgCl2]dic hloro-mercury (II)[Fe(SCN)3]tri-thiocyanato-iron (III)And finally, if they are written up with the counterions.

6 [Cu(NH3)4]SO4[tetraammine-copper(II)]-su lphateNa3[Ag(S2O3)2]sodium-[dithiocyanat o-argentate]Coordinated ligands always try to obtain positions around thecentral metal ion so that the compound becomes energeticallythe most stable that is the structure tends to be :Structure of Complex compoundsStructure of Complex compoundspcoordination number: 2structure is linearcoordination number: 4structure is tetrahedralcoordination number: 5structure is trigonal-bipiramidalcoordination number: 6structure is of Complex compoundsStructure of Complex compoundsComplex reactions always commence in a stepwise fashion.

7 Eachequilibrium is characterized by astepwise formation constant(the example below is given for a Complex with acoordination number of 4 and monodentate ligands): Complex formation equilibriaComplex formation equilibria[L][M][ML]K ML L M1 = +[L][ML]][MLK ML L ML 222 = +][ML[L]][ML][MLK ML L ML23332 = +[L]][ML][MLK ML L ML34443 = +where usually K1 K2 K3 K4. formation constant(K, ) for the last Complex inthe chain is the product of the formation constants.

8 = K1 K2 K3 K4 Complex formation equilibriaComplex formation equilibriannn[L][M]][ML ML L n M = + K1K2K3K4 Because of the parallel equilibria, there is a number of speciespresent in a solution containing Complex compounds ( M, L,MLML1ML2ML3ML4)Theconcentrationofthese speciesisML,ML1,ML2,ML3,ML4).Theconcentr ationofthesespeciesiscalculable, even if in a complicated manner, using the can also see that full complexation of a metal ion is onlypossible using a large excess of the formula for the overall formation constant suggest that thecomplex formation is only governed by the concentration of theligand, and the free metal ion.

9 For a hexadentate ligand (EDTA,Y4-)asanexampleitis:Conditional formation constantConditional formation constantY),asanexample,itis:In reality, the formation equlibrium is also affected by any sidereactions that influence [Y4-]or[Mn+]. These include: pH(sinceprotons compete with metal ions forthe ligand)p(ppg) other ligands(as they compete with ligand for the metal ion)These effects can be taken into account by introducing the socalledvirtual (conditional) formation anillustration, we will discuss the effect of pH in the ligand not complexed is present in various protonated we denote the total dissolved ligand concentration by Y , thenwe can writeConditional formation constantConditional formation constant[Y ] = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y]and we can define a proportionality factor: H = [Y ]/[Y4-]The value for this factor is, of course, calculable based on thestepwise dissociation equilibria for H4Y.

10 Ifwe do the math,wepq4,obtain: H = 1 + K1 [H+] + K1 K2 [H+]2+ K1 K2 K3 [H+]3 + K1 K2 K3 K3 [H+]4 Knowing the constants from K1to K4,the function can be plotted graphicallyagainst the formation constantConditional formation constantUsing this newly introduced factor wecan write:This means that we can calculate aconditional formation constant,which refers to the actually presenttotal ligand concentration, if we know H(as a function of pH). Complex compounds that are formed with multidentateligands in a way that five- or six-membered ringlike structuresare created around the metal ion, are very stable (see theexamplebelow)Theseringsarecalledchela teringsandtheChelate Complex formationChelate Complex formationexamplebelow).