1. AN INTRODUCTION TO TRANSITION METAL COMPLEXES KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

2. STABILITY CONSTANTS Definition The stability constant, K stab, of a complex ion is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions.

3. STABILITY CONSTANTS Definition The stability constant, K stab, of a complex ion is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions. In the reaction [M(H 2 O) 6 ] 2+ (aq) + 6X¯(aq) [MX 6 ] 4– (aq) + 6H 2 O(l)

4. STABILITY CONSTANTS Definition The stability constant, K stab, of a complex ion is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions. In the reaction [M(H 2 O) 6 ] 2+ (aq) + 6X¯(aq) [MX 6 ] 4– (aq) + 6H 2 O(l) the expression for the stability constant is K stab = [ [MX 6 4– ](aq) ] [ [M(H 2 O) 6 ] 2+ (aq) ] [ X¯(aq) ] 6

5. STABILITY CONSTANTS Definition The stability constant, K stab, of a complex ion is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions. In the reaction [M(H 2 O) 6 ] 2+ (aq) + 6X¯(aq) [MX 6 ] 4– (aq) + 6H 2 O(l) the expression for the stability constant is K stab = [ [MX 6 4– ](aq) ] [ [M(H 2 O) 6 ] 2+ (aq) ] [ X¯(aq) ] 6 The concentration of X¯(aq) appears to the power of 6 because there are six of the ions in the equation. Note that the water isn’t included; it is in such overwhelming quantity that its concentration can be regarded as ‘constant’.

6. STABILITY CONSTANTS Because ligand exchange involves a series of equilibria, each step in the process has a different stability constant… K stab / dm 3 mol -1 [Co(H 2 O) 6 ] 2+ (aq) + NH 3 (aq) [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + H 2 O(l) K 1 = 1.02 x 10 -2 [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + H 2 O(l) K 2 = 3.09 x 10 -2 [Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + H 2 O(l) K 3 = 1.17 x 10 -1 [ Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + H 2 O(l) K 4 = 2.29 x 10 -1 [Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 5 (H 2 O)] 2+ (aq) + H 2 O(l) K 5 = 8.70 x 10 -1 etc

7. STABILITY CONSTANTS Because ligand exchange involves a series of equilibria, each step in the process has a different stability constant… K stab / dm 3 mol -1 [Co(H 2 O) 6 ] 2+ (aq) + NH 3 (aq) [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + H 2 O(l) K 1 = 1.02 x 10 -2 [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + H 2 O(l) K 2 = 3.09 x 10 -2 [Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + H 2 O(l) K 3 = 1.17 x 10 -1 [ Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + H 2 O(l) K 4 = 2.29 x 10 -1 [Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 5 (H 2 O)] 2+ (aq) + H 2 O(l) K 5 = 8.70 x 10 -1 etc The overall stability constant is simply the equilibrium constant for the total reaction. It is found by multiplying the individual stability constants... k 1 x k 2 x k 3 x k 4... etc K stab or pK stab ?For an easier comparison, the expression pK stab is often used… pK stab = -log 10 K stab

8. STABILITY CONSTANTS Because ligand exchange involves a series of equilibria, each step in the process has a different stability constant… K stab / dm 3 mol -1 [Co(H 2 O) 6 ] 2+ (aq) + NH 3 (aq) [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + H 2 O(l) K 1 = 1.02 x 10 -2 [Co(NH 3 )(H 2 O) 5 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + H 2 O(l) K 2 = 3.09 x 10 -2 [Co(NH 3 ) 2 (H 2 O) 4 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + H 2 O(l) K 3 = 1.17 x 10 -1 [ Co(NH 3 ) 3 (H 2 O) 3 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + H 2 O(l) K 4 = 2.29 x 10 -1 [Co(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) + NH 3 (aq)[Co(NH 3 ) 5 (H 2 O)] 2+ (aq) + H 2 O(l) K 5 = 8.70 x 10 -1 etc Summary The larger the stability constant, the further the reaction lies to the right Complex ions with large stability constants are more stable Stability constants are often given as pK stab Complex ions with smaller pK stab values are more stable

9. REACTION TYPES The examples aim to show typical properties of transition metals and their compounds. One typical properties of transition elements is their ability to form complex ions. Complex ions consist of a central metal ion surrounded by co-ordinated ions or molecules known as ligands. This can lead to changes in... colour co-ordination number shape stability to oxidation or reduction Reaction types ACID-BASE LIGAND SUBSTITUTION PRECIPITATION REDOX A-B LS OX Ppt RED REDOX

10. REACTION TYPES The examples aim to show typical properties of transition metals and their compounds. LOOK FOR... substitution reactions of complex ions variation in oxidation state of transition metals the effect of ligands on co-ordination number and shape increased acidity of M 3+ over M 2+ due to the increased charge density differences in reactivity of M 3+ and M 2+ ions with OH¯ and NH 3 the reason why M 3+ ions don’t form carbonates amphoteric character in some metal hydroxides (Al 3+ and Cr 3+ ) the effect a ligand has on the stability of a particular oxidation state

11. REACTIONS OF COBALT(II) CO 3 2- [Co(H 2 O) 6 ] 2+ (aq) + CO 3 2- (aq) ——> CoCO 3 (s) + 6H 2 O (l) mauve ppt. Hexaaqua ions of metals with charge 2+ precipitate a carbonate but heaxaaqua ions with a 3+ charge don’t. Cl¯[Co(H 2 O) 6 ] 2+ (aq) + 4Cl¯ (aq) ——> [CoCl 4 ] 2- (aq) + 6H 2 O (l) pink, octahedral blue, tetrahedral Cl¯ ligands are larger than H 2 O Cl¯ ligands are negatively charged - H 2 O ligands are neutral the complex is more stable if tetrahedral - less repulsion between ligands adding excess water reverses the reaction LS Ppt

12. REACTIONS OF COPPER(II) Cl¯[Cu(H 2 O) 6 ] 2+ (aq) + 4Cl¯ (aq) ——> [CuCl 4 ] 2- (aq) + 6H 2 O (l) yellow, tetrahedral Cl¯ ligands are larger than H 2 O and are charged the complex is more stable if the shape changes to tetrahedral adding excess water reverses the reaction I¯ 2Cu 2+ (aq) + 4I¯ (aq) ——> 2CuI (s) + I 2 (aq) off - white ppt. a redox reaction used in the volumetric analysis of copper using sodium thiosulphate LS REDOX

13. REACTIONS OF MANGANESE(VII) in its highest oxidation state therefore Mn(VII) will be an oxidising agent occurs in the purple, tetraoxomanganate(VII) (permanganate) ion (MnO 4 ¯) acts as an oxidising agent in acidic or alkaline solution acidic MnO 4 ¯ (aq) + 8H + (aq) + 5e¯ ——> Mn 2+ (aq) + 4H 2 O (l) E° = + 1.52 V N.B. Acidify with dilute H 2 SO 4 NOT dilute HCl alkaline MnO 4 ¯ (aq) + 2H 2 O (l) + 3e¯ ——> MnO 2 (s) + 4OH¯ (aq) E° = + 0.59 V

14. VOLUMETRIC USE OF MANGANATE(VII) Potassium manganate(VII) in acidic (H 2 SO 4 ) solution is extremely useful for carrying out redox volumetric analysis. MnO 4 ¯ (aq) + 8H + (aq) + 5e¯ ——> Mn 2+ (aq) + 4H 2 O (l) E° = + 1.52 V It must be acidified with dilute sulphuric acid as MnO 4 ¯ is powerful enough to oxidise the chloride ions in hydrochloric acid. It is used to estimate iron(II), hydrogen peroxide, ethanedioic (oxalic) acid and ethanedioate (oxalate) ions. The last two titrations are carried out above 60°C due to the slow rate of reaction. No indicator is required; the end point being the first sign of a permanent pale pink colour. Iron(II)MnO 4 ¯ (aq) + 8H + (aq) + 5Fe 2+ (aq) ——> Mn 2+ (aq) + 5Fe 3+ (aq) + 4H 2 O (l) this means that moles of Fe 2+ = 5 moles of MnO 4 ¯ 1

15. CONTENTS REACTIONS OF IRON(II) VolumetricIron(II) can be analysed by titration with potassium manganate(VII) in acidic (H 2 SO 4 ) solution. No indicator is required. MnO 4 ¯ (aq) + 8H + (aq) + 5Fe 2+ (aq) ——> Mn 2+ (aq) + 5Fe 3+ (aq) + 4H 2 O (l) this means that moles of Fe2+ =5 moles of MnO 4 ¯ 1

16. © 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING THE END AN INTRODUCTION TO TRANSITION METAL COMPLEXES