Ionization Energy and HSAB Theory

• This section explores the inorganic chemistry of life, covering 5 lectures and 1 workshop.

Contact Information

• Dr Stephen Potts can be contacted at [email protected].
• Support hours are available on Mondays (appointment only) and Thursdays (drop-in, except January 30th) in the Christopher Ingold Building, room 133.

Recommended Reading

• Chemistry by Burrows et al. (4th edition, 2021), specifically Chapter 28, is recommended, with electronic access available.
• Inorganic Chemistry by Weller et al. (7th edition, 2018) is also listed, with a link for electronic access.
• Inorganic Chemistry by Housecroft & Sharpe (5th edition, 2018) is suggested and has a link for accessing it electronically.

Elemental Composition of Life

• Four organic base elements make up the majority of life.
• Quantity elements and essential trace elements are also required.
• Elements vary in abundance in the universe and on Earth and biological systems concentrate certain elements while rejecting others.
• Biological elements need to have a useful biological function, be abundant, and have an easily extractable form.
• The most abundant by mass in the human body are Oxygen (65%), Carbon (18%), Hydrogen 10% and Nitrogen (3%).

Section Intended Learning Outcomes

• Understand the periodic table and the abundance of elements.
• Consider elements available for life's experiment.
• Understand properties of elements (size and charge).
• Understand biological systems' element selection.
• Consider properties of metal ions and ligands and their binding.
• Understand the influence of oxidation states on binding properties.
• Understand relations between redox changes and conformational change.
• Relate hard and soft acid-base theory to metal complexes.
• Understand metal-based conformational preferences.

General Concepts

• First ionization energy, which relates to the energy required to create 1+ charged ions
• Oxidation states
• Lewis acids and bases
• Catalysis

Trends in First Ionization Energies

• Ionization energy increases across a period and decreases down a group.
• Nuclear charge, shielding, and effective nuclear charge influence ionization energy.
• As nucleus charge increases, the same is true for ionization energy.
• Elements are poorly shielding, therefore from Li to Be, the additional electron added to orbitals is poorly shielded.
• The decrease in E₁₁ down a group is due to the increase in atomic radius of the orbital involved, resulting in the electron being further from the nucleus and easier to remove.

Redox

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• Oxidation state is the of degree of oxidation of an atom.
• The oxidation state of a free element is zero.
• In simple ions, the oxidation state equals the net charge.
• In molecules, the oxidation states of an atom must be equal to that of the overall charge on the molecule.
• Electronegativity impacts the oxidation state for hydrogen, and periodic trends dictate electronegativity.
• Alkaline-earth elements are only restricted to +2 oxidation state.
• Group 12 elements zinc and cadmium exist only in the +2 oxidation state, meaning they are redox-inactive.

Lewis Acids and Bases

• Lewis acids are known to accept an electron pair.
• Lewis bases can donate an electron pair.
• Molecules with a high lying filled, and a low lying unfilled orbital can act as both a lewis acid and lewis base at the same time.
• Lewis acids react with Lewis bases to give acid-base pairs, known as adducts.

Coordination Compounds

• Coordination compounds contain a central metal atom or ion, and ligands that formally donate electrons to the metal.
• Ligands are atoms, molecules or ions that bonds to a metal centre.
• Coordination compounds are therefore Lewis acid-base adducts.
• A coordination sphere is the space around where the ligands bind.
• Denticity id the number of interactions a single ligand can make with the metal centre.
• The coordination number defines the number of metal-ligand interactions.

Ligand Types and Binding

• Atoms, ions, or molecules that binds to a central metal ion form a coordination complex.
• Ligands are classified as monodentate (one binding site) or polydentate (multiple binding sites).
• Two types of monodentate ligands, are neutral (ammine, aqua, carbonyl, thiocarbonyl, trialkylamine, trialkylphosphine, triphenylphosphine and pyridine) and anionic.
• A ligand that can coordinate to a metal in >1 ways are termed ambidentate ligands.
• The stability constant indicates interaction between metals and ligands.
• The hard-soft acid-base (HSAB) principle states hard acids form strong complexes with hard bases.

HSAB Classification

• Atomic/ionic radius affect the properties of a group
• Elements can therefore be considered:hard and soft acids/bases depending on the properties of the ligands
• In the HSAB principle, hard acid are small, difficult to polarise with a high charge density
• A soft base on the other hand as the opposite properties of hard acids

Catalysis

• In biological systems, Biological catalysts – proteins – generally show extreme selectivity (enzymes).
• The core of most enzymes are is a metal centre locked into a particular arrangement.
• Catalyst are defined as substance which takes part in a reaction and changes its rate but can be recovered unchanged at the end of the reaction.

Case Study 1: Valinomycin

• Sodium and potassium are essential components of all life and therefore regulated
• Valinomycin is a carrier transport catalyst, made of three repeat units; L-valine, D-hydroxy-isovaleric acid, D-valine and L-lactic acid.
• Bidentate ligands can displace monodentate ligands, this is known as the chelate ring.
• Formation of the chelate is the chelate effect, where a ~107 K value indicates that formation of the chelate complex is favourable.
• AS increases as monodentate ligands are replaced by bidentate ligands, since here are more molecules and therefore more degrees of freedom.
• Macrocyclic ligands have a greater stability constants for macrocyclic ligands, known as the macrocyclic effect.
• Alkali metals must be ligated by six water molecules in an octahedral arrangement before binding to ligands in an aqueous arrangement.
• Smaller atoms become harder based on a 30% difference in relative sizes. Valinomycin is 20,000 times more favourable when binding
• Binding of valinomycin and potassium enables the valinomycin to enter a lipid core.

Case Study 2: Carbonic Anhydrase (CA)

• Used to maintain acid-base balance in blood/tissues and transport CO2, and accelerate interconversion with bicarbonate.
• The active site most carbonic anhydrases use a Zn2+ ion and, Zn2+ is a strong Lewis acid.
• CO2 needs to be regulated because the bicarbonate anion, HCO3¯, is important in aqueous solution.
• Carbonic anhydrases can catalyse both the removal, and generation of CO2.
• The enzyme is a genetically and immunologically distinct cytosolic isozymes which are known.
• Water + high ionisation results in a weakening of the O-H bond and liberation of a proton.
• The metal bound hydroxide from the bound water can then attack CO2 leading to formation of bicarbonate in a four step process.

Case Study 3: Superoxide Dismutase (SOD)

• SOD catalyses the dismutation of superoxide into oxygen and hydrogen peroxide, in an antioxidative defence role where O₂- is a radical which impacts cells.
• SOD has three Co-factored forms of metal, in which metal has differing effect.
• SOD1 (Cu/Zn) is found in humans.
• Cu2+ and Cu⁺ have an equal tendency to form complexes with organic molecules.
• The superoxide and copper centre need to come into direct contact once for electron transfer to occur.
• It has been discovered that the O2 has oxidation state number of -0.5 (as appose to oxide which is -2).
• It has also been shown Cu2+ and Zn2+ can be removed from enzymes.
• Metal ions can be linked via linked via a His residue.
• The zinc ion is crucial for maintaining and electrostatic support of zinc bending loops The mutation cause decline of zinc causes motor disease, which results in death.
• SOD active sight has three main steps:
  • The species come into intimate contact, where a bond exits between the reactants in the transition state.
  • Getting protons to the active site: proton channels. -A high Kubas interaction and diffusion result in H+H formation
  • The active site is - H cluster - why CO and CN- ligands?

Case Study 4: Hydrogenases

• These enzymes catalyse the reversible oxidation of molecular hydrogen and plays a pivotal role in anaerobic metabolism.
• The active metal core of hydrogenases is a metallocluster stabilised by CO and CN– ligands, in an electron transfer clusters like ferredoxins.
• Fe-H2ase involves many number of process: hydrothermal vents, transfer of electrons and bacterial production of enzymes.
• High electronegativity of oxygen means that metal centres bound in CO and CN have low stability therefore low oxidation.
• CO is highly synergic so binds well in both donation and reciprocation.
• One ion atom has a weakly bound water molecule, while the second is linked via a protein.
• One iron acts as a redox centre, while the other acts as a proton bending sight.

Explore ionization energy trends across the periodic table, focusing on effective nuclear charge and shielding. Investigate the Hard Soft Acid Base (HSAB) theory, examining how ionic radius and charge density influence interactions between acids and bases.