Review of Inorganic Chemistry

1. Review of Inorganic Chemistry • Acid-base Chemistry • Coordination Chemistry • Valence Bond Theory • Crystal Field Theory • Molecular Orbital Theory • Valence Shell Electron-pair repulsion • Transition Metal Complexes • Coordination number • Stereochemistry • Kinetics of Ligand Exchange • Mechanisms of Ligand Substitution Reactions • Mechanisms of Redox Reactions

2. Acid-Base Concepts We are familiar with the concept of acids as proton donors Can there still be acids and bases if no protons are involved ?

3. Determination of Ligand pK Values The capacity of ligands to bind metal ions depends on their ability to act as electron pair donors The carboxyl group is a potential metal ion ligand even at low pH The amino group has a lone pair of electrons to interact with a metal ion at higher pH (Nelson & Cox: Lehninger Principles of Biochemistry, 3rd ed.)

4. Effects of Metals on pK Values Does the presence of metal ions alter ligand pK values ? Metal ions shifts the deprotonation equilibrium, and therefore the pK values The magnitude of the shift is determined by the affinity of metal ion binding

5. Hard-Soft Acid-Base Classifications What determines the affinity between metals and ligands ? The most stable complexes are formed between metals and ligands with similar properties (hard with hard and soft with soft)

6. Properties of Hard and Soft Metal Ions Hard metal ions tend to have low ionization energies (more likely to lose electrons) Soft metal ions are more polarizable

7. Coordination ChemistryBonding Theories

8. Coordination Chemistry The structure of a coordination complex will be determined by: 1. coordination number 2. geometry 3. reactivity • In general, the lowest overall energy (greatest stability) will be achieved in a coordination compound by: • forming the maximum number of bonds • forming the strongest possible bonds • arranging the ligands to minimize adverse repulsive energies

9. Coordination Geometries the number and type of orbitals involved in ligand binding determines the preferred coordination geometry

10. Stereochemistry and Oxidation States of 3d elements

11. Ligand-field Splittingeffect of coordination geometry These orbital diagrams are useful to correlate the properties of metal centers in proteins such as optical spectra and magnetic properties with their structures and reactivities

12. Ligand-field Splitting • The strength of the ligand field influences the orbital splitting (Δ), which is the energy separation between orbitals • This is determined by the nature of the donor atoms bonded to the metal ion • This order is called the spectrochemical series: The stronger ligands (e.g. CN or CO) lead to greater energy differences between the orbitals

13. Ligand-field Splittingeffect on coordination geometry Ni(II) complexes (d8) can exist in several coordination geometries A diamagnetic Ni(II) complex will have square planar geometry A paramagnetic Ni(II) complex will prefer tetrahedral geometry unless a strong ligand field leads to greater orbital splitting

14. Coordination Geometries • For simple metal-ligands complexes the coordination number and coordination geometry is dictated by the requirements of the metal ion • For biological ligands there are frequently multiple groups within the ligand that can function as donor atoms to the metal ion • The geometric constraints of these multidentate ligands will frequently influence the overall geometry and the properties of the complex

15. Ligand Exchange Kinetics How rapidly do ligands dissociate from a metal complex ? Why do some metal ions exchange ligands in seconds while others take days for the same exchange?

16. Ligand Exchange Kinetics

17. Ligand Exchange Kinetics Ligand exchange rates are generally faster for less charged cations: M+ > M2+ > M3+ Neutral ligands typically exchange more rapidly than charged ligands Second and third row transition metal complexes are more kinetically inert than those of the corresponding first row metals

18. Factors that influence the lability of metal complexes • Changes in conditions • pH, temperature, ionic strength • Changes in ligands • neutral vs. charged ligands • monodentate vs. multidentate • Changes in accessibility • buried vs. solvent exposed

19. Metal Ion Chelation chelate: multiple donor atoms from a single ligand coordinating a metal ion In order for the metal ion to dissociate from EDTA the bonds to each of the donor atoms must be broken This decreases the lability and increases the stability of the complex

20. Ligand Substitution Mechanisms The exchange of ligands to a metal ion can occur by two general mechanisms The bond to the incoming ligand forms before the departing ligand leaves The bond to the departing ligand breaks before the incoming ligand arrives How can you distinguish between the two possible mechanisms ?

21. Supporting Evidence for a Dissociative Mechanism The strength of the metal-ligand bond in inversely proportional to the rate of the reaction Electron donating ligands increase the reaction rate Increasing the total negative charge on the complex increases the reaction rate

22. Supporting Evidence for an Associative Mechanism The reaction rate depends on the nature or the concentration of the entering group Bulkier ligands in the complex tend to slow the reaction rate

23. Ligand tuning of metal redox potentials redox potential: a measure of the likelihood of electron transfers to and from a metal complex • Cu(I) prefers tetrahedral coordination, while Cu(II) complexes are typically square planar • Ligands that prefer tetrahedral geometry will stabilize the +1 oxidation state, making Cu(II) a better oxidant and thus raising the redox potential • Bulkier substituents cause more distortion in the planar ligand, leading to higher redox potentials

24. Ligand tuning of metal redox potentials • Cu(I) is a soft acid and prefers binding to soft ligands • Charging the donor atom from O to S favors Cu(I), thereby increasing the redox potential • Biological macromolecules can also use functional groups in the vicinity of the bound metal to influence the redox potential by perturbing the local dielectric constant

25. Summary • Metal ions can cause substantial shifts in ligand pK values • The most stable complexes are formed between like metals and ligands • The nature of the donor atoms will affect ligand field splitting • Coordination number and geometry are dictated by the metal ion, but are influenced by the ligands • Ligand exchange rates are fastest for less charged metal ions • Ligands can fine tune metal redox potentials