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CHEM1612 - Pharmacy Week 8: Complexes II

CHEM1612 - Pharmacy Week 8: Complexes II. Dr. Siegbert Schmid School of Chemistry, Rm 223 Phone: 9351 4196 E-mail: siegbert.schmid@sydney.edu.au. Unless otherwise stated, all images in this file have been reproduced from:

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CHEM1612 - Pharmacy Week 8: Complexes II

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  1. CHEM1612 - Pharmacy Week 8: Complexes II Dr. SiegbertSchmid School of Chemistry, Rm 223 Phone: 9351 4196 E-mail: siegbert.schmid@sydney.edu.au

  2. Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, John Wiley & Sons Australia, Ltd. 2008      ISBN: 9 78047081 0866

  3. Complexes • Blackman, Bottle, Schmid, Mocerino & Wille Chapters 13,10.4, 11.8 • Complex ions • Coordination compounds • Geometry of complexes • Chelates • Kstab • Solubility and complexes • Nomenclature • Isomerism in complexes • Biologically important metal-complexes Co(EDTA)-

  4. Complex Formation and Solubility • Example:AgBr(s) Ag+(aq) + Br-(aq) • Calculate the solubility of AgBr in: a) water b) 1.0 M sodium thiosulfate (Na2S2O3) (Ksp (AgBr)= 5.0·10-13, Kstab ([Ag(S2O3)2]3- )= 4.7·1013 ) a) Solubility of AgBr in water Ksp = [Ag+][Br-] AgBr(s) Ag+(aq) + Br-(aq) x x Ksp = x2 = 5.0·10-13x = 7.1 ·10-7 M

  5. [Ag(S2O3)23-][Br-] [S2O32-]2 b) Solubility of AgBr in sodium thiosulfate 1.0 M Na2S2O3 (1) AgBr(s) Ag+(aq) + Br-(aq) Koverall = Ksp x Kstab = = 5.0·10-13 x 4.7·1013 = 24 Ag+(aq) + 2S2O32-(aq) [Ag(S2O3)2]3-(aq) AgBr(s) + 2S2O32-(aq) [Ag(S2O3)2]3-(aq) + Br-(aq) (2) (1)+(2) 0 +x x 1.0 M -2x 1.0 -2x Initial Conc. Change Equilibrium Conc. 0 +x x Substitute: Koverall = x2/(1.0 - 2x)2 = 24 x = 0.45 Solubility of AgBr in thiosulfate is 0.45 M (c.f. in water 7.1 x 10-7 M)

  6. Nomenclature I Rules for nomenclature of coordination compounds: • Name cation, then anion, as separate words. Examples: [Pt(NH3)4Cl2](NO2)2 tetraamminedichloridoplatinum(IV) nitrite [Pt(NH3)4(NO2)2]Cl2 tetraamminedinitritoplatinum(IV) chloride • Name the ligands then the metal, all in same word. • Number of ligands as Greek prefixes (di-, tri-, tetra-, penta-, hexa-), except ligands that already have numerical prefixes which use Latin prefixes (bis, tris, tetrakis…) • e.g. bis(ethylenediamine) for (en)2

  7. Nomenclature II • Oxidation state in Roman numeral in parentheses after name of metal • e.g. [Ag(NH3)2]NO3diamminesilver(I) nitrate • Anionic ligands end in '-ido‘: • Neutral ligands named as molecule, except those listed here: Ligand NameFormula Fluorido F- ChloridoCl- Bromido Br- Iodido I- Cyanido CN- Hydroxido OH- (Please modify accordingly pp.518-519 of your book) Ligand NameFormula Ammine NH3 Aqua H2O Carbonyl CO Nitrosyl NO

  8. Nomenclature of Ligands • Ligands named in alphabetical order (but prefixes do not affect the order) • e.g. [Co(NH3)5Cl]SO4 pentaamminechloridocobalt(III) sulfate • Anionic complexes end in ‘-ate’ • e.g. K3[CrCl6] potassium hexachloridochromate(III) • Some metals in anionic complexes use Latin -ate names: Not Ironate Not Copperate Not Leadate Not Silverate Not Goldate Not Tinnate

  9. Nomenclature - Exercises • [Co(H2O)6]CO3 hexaaquacobalt(II) carbonate • [Cu(NH3)4]SO4 tetraamminecopper(II) sulfate • (NH4)3[FeF6] ammonium hexafluoridoferrate(III) • K4[Mn(CN)6] potassium hexacyanidomanganate(II)

  10. Assigning oxidation numbers • Example 1: Find O.N. of Co in : [Co(NH3)5Cl]SO4pentaamminechloridocobalt(?) sulfate [Co(NH3)5Cl]2+ ammine is neutral, chloride is -1 O.N. -1 = +2 (sum of O.N.s = overall charge) O.N. = +3 • Example 2: Find O.N. of Mn in :K4[Mn(CN)6] potassium hexacyanidomanganate(?) [Mn(CN)6]4- (CN) is -1 overall O.N. + 6x(-1) = -4 (sum of O.N.s = overall charge) ON = +2

  11. Isomerism in Complexes Complexes can have several types of isomers: • Structural Isomers: different atom connectivities • Coordination sphere isomerism • Linkage isomerism • Stereoisomers: same atom connectivities but different arrangement of atoms in space • Geometric isomerism • Optical isomerism

  12. Coordination Isomers • Ligands and counter-ions exchange place: Example: • [Pt(NH3)4Cl2](NO2)2 tetraamminedichloridoplatinum(IV) nitrite • [Pt(NH3)4(NO2)2]Cl2 tetraamminedinitritoplatinum(IV) chloride • Two sets of ligands are reversed: [Cr(NH3)6][Co(CN)6] NH3 is a ligand for Cr3+ [Co(NH3)6][Cr(CN)6] NH3 is a ligand for Co3+ ligands counterions

  13. cyanato NCO:→ isocyanato OCN:→ cyanate ion Linkage isomers • Occur when a ligand has two alternative donor atoms. thiocyanatoNCS:→ isothiocyanatoSCN:→ thiocyanateion Pentaammineisothiocyanatocobalt(III) pentaamminethiocyanatocobalt (III)

  14. Stereoisomers: Geometric Isomers Square planar complex. Four coordinate: cis- and trans-[Pt(NH3)2Cl2] Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. cisplatin – highly effective anti-tumour agent No anti-tumour effect

  15. Stereoisomers: Geometric Isomers Octahedral complex. Six coordinate: cis- and trans- [Co(NH3)4Cl2]+ 2 Cl next to each other violet 2 Cl axial to each other green

  16. Stereoisomers: Optical Isomers • When a molecule is non-superimposable with its mirror image. • Example: four different substituents about tetrahedral centre. • Same physical properties, except direction in which they rotate the plane of polarized light. [NiClBrFI]2-

  17. cis-[Co(NH3)4Cl2]+ cis-[Co(en)2Cl2]+ + + Has no optical isomers Has optical isomers Stereoisomers: Optical isomers • Metal atoms with tetrahedral or octahedral geometries (but not square planar) may be chiral due to having different ligands. • For the octahedral case, several cases are possible, e.g. • Complex with four ligands of two types.

  18. [M(en)3]n+ complexes have optical isomers: Not superimposable 3+ 3+ Mirror plane Stereoisomers: Optical isomers Having three bidentate ligands of only one type - gives a propeller-type structure. www.pt-boat.com

  19. Octahedral complex - stereoisomerism Cis- Dichlorido Bis(ethylendiamine)cobalt(III) ion Mirror image Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. rotation of I by 180°gives III ≠ II

  20. Octahedral complex - stereoisomerism Trans- Dichlorido Bis(ethylendiamine)cobalt(III) ion Mirror image Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. rotation of I by 270°gives III = II

  21. Biologically Important Complexes • Many biomolecules contain metal ions that act as Lewis acids. Give some examples of naturally occurring complexes. • Heme • Chlorophyll • Vitamin B12 • Enzyme Carbonic anhydrase

  22. Heme O2 bound to Fe2+ Heme is a square planar complex of Fe2+ and the tetradentate ring ligand porphyrin (bonds to 4 donor N atoms). Present in hemoglobin, which carries oxygen in blood, and myoglobin, which stores oxygen in muscle. Porphyrin ring Myoglobin protein Blackman Figure 13.37

  23. Chlorophyll • Chlorophyll is a photosynthetic pigment, that gives leaves the characteristic green colour. It is a complex of Mg2+ and a porphyrin ring system (four N atoms are the chelae). Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

  24. Vitamin B12 Dorothy Crowfoot Hodgkin The Nobel Prize in Chemistry 1964 Nobelprize.org Image download from Wikipedia

  25. Tetrahedral complex of Zn2+. Catalyses reaction between water and carbon dioxide during respiration. Coordinated to 3 N, fourth site left free to interact with molecule whose reaction is being catalysed (here with water). By withdrawing electron density, makes water acidic to lose proton and OH- attacks partial positive C of CO2 much more vigorously. Cd2+ is toxic because it competes with zinc for this spot. Carbonic anhydrase Figure downloaded from Wikipedia CO2(g) + 2H2O(l) H3O+(aq) + HCO3-(aq)

  26. Summary • Concepts: • Complex formation • Stability constant and stepwise stability constant • Acidity of some metal ions in solution • Coordination compounds and geometry • Nomenclature of coordination compounds • Isomerism in Complexes • Calculations • Complex Formation • Equilibria in solution: complex formation + solubility

  27. H N Br 3 N H Br 3 Pt Pt Cl N=N=N Cl N=N=N Question • Does the square planar complex ion [Pt(NH3)(N3)BrCl]- have optical isomers? This complex has no optical isomers because it can be superimposed on its mirror image.

  28. Complex Formation and solubility • Metal complex formation can influence the solubility of a compound. e.g. AgCl(s) + 2 NH3 [Ag(NH3)2]+ + Cl- • This occurs in 2 stages: AgCl(s) Ag+ + Cl- (1) Ag+ + 2 NH3 [Ag(NH3)2]+ (2) • Complex formation removes the free Ag+ from solution and so drives the dissolution of AgCl forward.

  29. [Ag(NH3)2+][Br-] [NH3] Solubility of AgBr in Ammonia 1.0 M NH3 Kstab(Ag(NH3)2+)= 1.7·107) (1) AgBr(s) Ag+(aq) + Br-(aq) Koverall = Ksp x Kstab = = 5.0·10-13 x 1.7·107 = 8.5·10-6 Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+(aq) AgBr(s) + 2NH3(aq) [AgNH3]+(aq) + Br-(aq) (2) (1)+(2) 0 +x x 1.0 M -2x 1.0 - 2x Initial Conc. Change Equilibrium Conc. 0 +x x Substitute: Koverall = x2/(1.0-2x)2 = 8.5·10-6 x = 2.9·10-3 M Solubility of AgBr in NH3 is 2.9·10-3 M (c.f. in thiosulfate 0.45 M)

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