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CHEM1612 - Pharmacy Week 8 : Complexes I

CHEM1612 - Pharmacy Week 8 : Complexes I. 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 I

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  1. CHEM1612 - Pharmacy Week 8: Complexes I 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. [M(H2O)4]2+ adduct M2+ H2O(l) Metal Ions as Lewis Acids • Whenever a metal ion enters water, a complex ion forms with water as the ligand. • Metal ions act as Lewis acid (accepts electron pair). • Water is the Lewis base (donates electron pair). +

  5. Complex Ions • Definition: A central metal ion covalently bound to two or more anions or molecules, called ligands. • Neutral ligands, e.g., water, CO, NH3 • Ionic ligands, e.g., OH-, Cl-, CN- [Ni(H2O)6]2+, a typical complex ion: • Ni2+ is the central metal ion • Six H2O molecules are the ligands • O are the donor atoms • overall 2+ charge.

  6. Coordination Compounds • They consist of: • Complex ion (metal ion with attached ligands) • Counter ions (additional anions/cations needed for zero net charge) • e.g. [Co(NH3)6]Cl3 (s) [Co(NH3)6]3+(aq) + 3 Cl-(aq) Complex ion Counter ions In water coordination compounds dissociate into the complex ion (cation in this example) and the counterions (3 Cl- ions here). Note: the counter ion may also be a complex ion. e.g. [Co(H2O)6][CoCl4]3(s) [Co(H2O)6]3+(aq) + 3 [CoCl4]-(aq)

  7. Acidic solution Acidity of Aqueous Transition Metal Ions A small and multiply-charged metal ion acts as an acid in water, i.e. the hydrated metal ion transfers an H+ ion to water. 5 bound H2O molecules 1 bound OH- (overall charge reduced by 1) 6 bound H2O molecules Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

  8. ACID STRENGTH Metal Ion Hydrolysis Each hydrated metal ion that transfers a proton to water has a characteristic Ka value.

  9. Coordination Number • The number of ligand atoms attached to the metal ion is called the coordination number. • varies from 2 to 8 and depends on the size, charge, and electron configuration of the metal ion. • Typical coordination numbers for some metal ions are: M+ Coord no. M2+Coord no. M3+Coord no. Cu+ 2,4 Mn2+ 4,6 Sc3+ 6 Ag+ 2 Fe2+ 6 Cr3+ 6 Au+ 2,4 Co2+ 4,6 Co3+ 6 Ni2+ 4,6 Au3+ 4 Cu2+ 4,6 Zn2+ 4,6

  10. Coordination Number and Geometry

  11. Ligands • Ligands must have a lone pair to donate to the metal. • The covalent bond formed is sometimes referred to as a “dative” bond. • Ligands that can form 1 bond with the metal ion are called monodentate (denta – tooth) e.g. H2O, NH3, Cl- (a single donor atom). • Some ligands have more than one atom with lone pairs that can be bonded to the metal ion – these are called CHELATES (greek: claw). • Bidentateligands can form 2 bonds e.g. Ethylenediamine • Polydentateligands – can form more than 2 bonds • For a list of ligands see the recommended textbook.

  12. BidentateChelateLigands MX+(en) Ethylenediamine (en) has two N atoms that can form a bond with the metal ion, giving a five-memberedring. Blackman, Bottle, Schmid, Mocerino & Wille, Figure 13.10

  13. Demo: Nickel Complexes Ni2+ forms three complexes with ethylenediamine: • Mix [Ni(H2O)6]2+ and en in ratio 1:1 → [Ni(en)(H2O)4]2+ greenblue-green • Mix [Ni(H2O)6]2+ and en in ratio 1:2 → [Ni(en)2(H2O)2]2+ light blue • Mix [Ni(H2O)6]2+ and en in ratio 1:3 → [Ni(en)3]2+purple

  14. Hexadentate ligand: EDTA Ethylenediaminetetraacetate tetraanion (EDTA4-) EDTA forms very stable complexes with many metal ions. EDTA is used for treating heavy-metal poisoning, because it removes lead and other heavy metal ions from the blood and other bodily fluids. Co(III) N=blue O=red [Co(EDTA)]-

  15. 3 more steps 3NH3 Lewis bases: water and ammonia The stepwise exchange of NH3 for H2O in M(H2O)42+. NH3 M(H2O)42+ M(H2O)3(NH3)2+ Ammonia is a stronger Lewis base than water M(NH3)42+ Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

  16. Equilibrium Constant Kstab Metal Ion + nLigand Complex • The larger Kstab, the more stable the complex, e.g. The complex formation equilibrium is characterised by a stability constant, Kstab(also called formation constant): Ag+(aq) + 2 NH3 Ag(NH3)2+(aq)

  17. Stepwise Stability Constant • Metal ions gain ligands one at a time. • Each step characterised by a specific stability constant. • Overall formation constant: Kstab = K1 x K2…x Kn • Example: Ag+(aq) + NH3(aq)[Ag(NH3)]+(aq) K1 = 2.1 · 103 [Ag(NH3)]+(aq) + NH3(aq)[Ag(NH3)2]+(aq) K2 = 8.2 · 103 Ag+(aq) + 2 NH3(aq)[Ag(NH3)2]+(aq)Kstab= ?

  18. 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)

  19. [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 = = Ag+(aq) + 2S2O32-(aq) [Ag(S2O3)2]3-(aq) AgBr(s) + 2S2O32-(aq) [Ag(S2O3)2]3-(aq) + Br-(aq) (2) (1)+(2) Initial Conc. Change Equilibrium Conc.

  20. The One Pot Reaction Ag+(aq) + OH-(aq)AgOH(s) brown Ksp = AgOH(s) + H2PO4-(aq) Ag3PO4(s) yellow Ksp = Ag3PO4(s) + HNO3(aq) Ag+(aq) + H3PO4(aq) Ag+(aq) + Cl-(aq) AgCl(s) white Ksp = AgCl(s) + 2 NH3(aq) [Ag(NH3)2]+(aq) + Cl-(aq) Kstab = [Ag(NH3)2]+(aq) + Br-(aq) AgBr(s) (green/white) Ksp = AgBr(s) + 2 S2O32-(aq) [Ag(S2O3)2]3-(aq)+Br-(aq) Kstab = [Ag(S2O3)2]3-(aq) + I-(aq) AgI(s) (yellow) Ksp = AgI(s) + 2 CN-(aq) [Ag(CN)2]-(aq) + I-(aq) Kstab = [Ag(CN)2]-(aq) + S2-(aq) Ag2S(s) (black) Ksp = * Note: Not all reaction equations are balanced

  21. Additional Exercise 0.01 moles of AgNO3 are added to a 500 mL of a 1.00 M solution of KCN. Then enough water is added to make 1.00 L of solution. Calculate the equilibrium [Ag+] given Kstab [Ag(CN)2]– =1020 M–2. (careful with the direction of the equation represented by Kstab!) Ag+ + 2CN– [Ag(CN)2]– initial /M 0.01 0.500 0 change ~ -0.01 -0.02 0.01 equilibrium /M x 0.480 0.01

  22. Solubility of AgBr in Ammonia 1.0 M NH3 Kstab(Ag(NH3)2+)= 1.7·107 (1) AgBr(s) Ag+(aq) + Br-(aq) Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+(aq) AgBr(s) + 2NH3(aq) [AgNH3]+(aq) + Br-(aq) (2) (1)+(2) Initial Conc. Change Equilibrium Conc.

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