H - Bio-Inorganic Chemistry Flashcards
What is the first ionisation energy?
The energy required to remove the most loosely held electron from each atom in a mole of gaseous atoms to produce one mole of gaseous ions each with a charge of 1+.
X(g) → X+(g) + e-
What are the trends in ionisation energy in the Periodic Table?
First ionisation energy generally increases along a period because electrons are added into the same energy level. Electrons in the same energy level are poorly shielding, so as electrons are added from left to right they poorly shield the extra nuclear charge resulting from the additional protons. This means the effective nuclear charge felt by the valence electrons increases, hence increasing the first ionisation energy.
First ionisation energy decreases down a group because the atomic radius of the orbital involved increases dramatically. This means that the most loosely held electron is further from the nucleus and so easier to remove.
What is the difference between a Lewis acid and a Lewis base?
Lewis acid → species that have an unoccupied orbital which can accept a pair of electrons ie. an electron-pair acceptor
eg. H+, BH3, AIF3, SF4
Lewis base → species that have a pair of electrons not used in bonding, usually a lone pair ie. an electron-pair donor
eg. H-, NH3, PMe3, F-
What is synergic bonding?
When a molecule with a high-lying filled orbital as well as a low-lying unfilled orbital acts as a Lewis acid and a Lewis base at the same time.
This happens in many ligands, including CO.
What is the stability constant, and what can its position be used to classify?
The position of equilibrium can be defined by the equilibrium or stability constant
The position of equilibrium can be used to classify Lewis acids and Lewis bases into two types: hard and soft.
What is the difference between hard and soft acid/bases?
Hard acids and bases tend to have a smaller ionic radius, a high oxidation state and weak polarizability.
Soft acids and bases tend to have a larger ionic radius, a low oxidation state and strong polarizability.
Hard acids preferentially react with hard bases and soft acids with soft bases.
The interaction between a hard acid and a hard base is mostly ionic, whereas the interaction between a soft acid and a soft base is mostly covalent.
How are acids and bases classified?
Going down a group in the Periodic Table means the atomic radius increases, so the species is getting bigger and the electrons more polarizable ie. they become softer.
The hardness of a metal ion (with variable oxidation state) increases with increasing charge because the ionic radius decreases, the species gets smaller and electrons become less polarizable.
What is a catalyst and how do they work?
A catalyst is a substance that takes part in a reaction and changes its rate but which can be recovered unchanged at the end of the reaction.
A catalyst provides the reaction with a separate mechanism or pathway which has a lower activation energy than the unassisted pathway.
What is a ligand?
A ligand is an atom, ion or molecule that binds to a central metal ion to form a coordination complex.
When this happens, the metal ion acts as a Lewis acid and the ligand acts as a Lewis base.
What is chelating?
When a bidentate ligand displaces two monodentate ligands, thereby chelating the metal ion and forming a chelate ring.
What is the chelate effect?
Metal ions have an enhanced affinity for chelating ligands rather than a collection of similar monodentate ligands. This is measured using stability constants.
The chelate effect is a thermodynamic effect. As the bonds between the metal ion and the different ligands are very similar, ΔH will tend towards zero.
ΔS increases as we replace monodentate ligands with bidentate ligands because this produces more molecules, and hence, there are more degrees of freedom.
How is the chelate effect relevant to medicine?
Chelation therapy uses chelating agents to detoxify poisonous metal agents such as mercury, arsenic and lead. This converts them into a chemically inert form that can be excreted from the body.
Chelating ligands are used in a similar way to remove excess iron from the body resulting from chronic transfusion therapy.
Chelate complexes of gadolinium are used as contrast agents in MRI scanning.
What is the macrocyclic effect?
Stability constants for macrocyclic ligands are greater than those for the analogous acyclic ligand.
This is because macrocyclic ligands only have a small loss in entropy upon binding to the metal ion because the ligand was rigid to begin with.
However, when the acyclic ligand binds there is a large loss in entropy because it goes from being very flexible to very rigid.
