Schedule

2. Schedule • Lecture 7: M-M bondsd-bonds and bonding in metal clusters • Lecture 8: Rates of reaction Ligand-exchange reactions, labile and inert metal ions • Lecture 9: Redox reactions Inner and outer-sphere reactions

3. Summary of Last Lecture Quadruple bonds • Eclipsed geometry due to d-bond Clusters • Consider total number of pairs of metal electrons and number of metal-metal connections • Bond order is number of pairs / number of edges Carbonyl clusters • Use 18 e- rule to work out how many bonds have to be formed Today’s lecture • Ligand-substitution reactions

4. Recap - Kinetics transition state H‡ (backward) H‡ (forward) DG (forward) = -DG (backward)

5. Recap - Kinetics • In the majority of reactions a series of such events occurs • in this case, each event is called a elementary step • the collection of these is the stoichiometric or reaction mechanism • the elementary step with the largest activation energy is called the rate determining step (or rds) • The reaction can only proceed if the reactants have at least this energy • The details of the rds are known as the intimate mechanism.

6. Ligand Substitution Reactions • In these reactions, one ligand is exchanged for another • Common examples include: • Aquation or acid hydrolysis: substitution of a ligand by H2O [Ni(en)3]2+ + H2O  [Ni(en)2(H2O)]2+ • Anation: substitution of H2O by an anion [Co(NH3)5(H2O)]2+ + Cl- [Co(NH3)5Cl]+ • Two stoichiometric mechanisms need to be considered: L5M + X + Y dissociatve (d) L5MX + Y L5MY + X L5MXY associatve (a)

7. k1 k2 Dissociative (d) Mechanisms • If the stoichiometric mechanism is dissociative, there are again two possible intimate mechanisms • D mechanism • M-X bond breaks before Y attaches L5MX L5M + X • 5-coordinate intermediate can, in theory, be isolated • equivalent to SN1 k-1 L5M + Y L5MY • Interchange Id mechanism • Before M-X bond fully breaks,M-Y bond begins to form • No intermediate • equivalent to SN2 M Y X

8. k1 k2 Associative (a) Mechanisms • If the stoichiometric mechanism is associative, there are two possible intimate mechanisms • A mechanism • M-Y bond forms before M-X bond breaks L5MX + Y L5MXY • 7-coordinate intermediate can, in theory, be isolated k-1 L5MXY L5MY + X • Interchange Ia mechanism • Before M-Y bond is fully made,M-X bond begins to break • No intermediate • equivalent to SN2 M Y X X

9. M Y X M Y X Interchange Mechanisms: Idvs Ia • Both the Id and Ia mechanism involve the interchange of X and Y without a definite intermediate • Id mechanism • In the transition state, bond breaking is more important than bond making • The activation energy is determined by M-X bond strength • Interchange Ia mechanism • In the transition state, bond formation is more important than bond breaking • Activation energy is determined by the crowding in the transition state

10. Water Exchange • The simplest substitution is the exchange of coordinated water with the solvent. • Rate varies over at least 16 orders of magnitude depending on the metal. rateconstant (s-1) very slow: kinetically inert fast: kinetically ‘labile’

11. Water Exchange • Activation energy for dissociate mechanisms (D and Id) determined by: • M-X bond strength M+-X weaker than M2+-X so M+ faster M2+-X weaker than M3+-X so M2+ faster The larger Mn+ is, the weaker its bonds so faster • Change in LFSE between 6-coordinate L5MX and 5 coordinate L5M • Insensitive to nature of nucleophile Y • Activation energy for associative mechanisms (A and Ia) determined by: • Crowding in 7-coordinate transition state Small Mn+ give ore crowded transition states and slower reactions • Change in LFSE between 6-coordinate L5MX and 7 coordinate L5MXY • Sensitive to nature of nucleophile Y

12. 0 1 2 3 4 5 6 7 8 9 10 Water Exchange for Transition Metals M3+ exchange water slower than M2+ whether mechanism as d or a • M3+ are smaller than M2+ • M3+-X are stronger than M2+-X slow for d3 and d8 andlow spin d4, d5, d6, d7 • Loss of LFSE is greatest for dn with large LFSE -LFSE / oct 2.4 -+0.6 Doct 1.2 -0.4 Doct

13. Water Exchange for Cu2+ and Cr2+ • These two ions are much more labile than their charge and LFSE indicates • Cr2+ (d4) • Cu2+ (d9) Jahn-Teller distortion: long axial bonds are weak and easily replaced AJB – lecture 1, JKB – Lecture 6

14. Experimental Tests of Mechanism • Dissociative mechanisms (D or Id): • H‡ large (bond breaking) • S‡ large and positive (increasing number of molecules) • V‡ large and positive (increasing number of molecules) • Insensitive to nucleophile • Highly sensitive to leaving group rds L5MX L5M + X rds • Associative mechanisms (A or Ia): • H‡ small (no bonds broken) • S‡ large and negative (decreasing number of molecules) • V‡ large and negative (decreasing number of molecules) • Sensitive to nucleophile • Insensitive to leaving group L5MX + Y L5MXY

15. Square Planar Complexes • For square planar complexes, substitution occurs via an A mechanism • it is stereospecific cis-platin cis product • H‡ small • S‡ large and negative • V‡ large and negative

16. Summary By now you should be able to.... • Describe the steps in the dissociative and associative mechanisms • Account for the wide variation in the rates of ligand exchange for • metals of different oxidation states and • metals of different dn configuration Next lecture • e- transfer reactions

17. Practice