Cation receptors Flashcards
Why is H+ a very strong Lewis acid?
Very small size so has a high charge density
H+ in HSAB sense
Hard Lewis acid so has high affinity for hard ligands (Lewis bases) such as F-, OH- and NH3
But virtually no affinity for soft halide ions e.g. Cl-, Br- and I-
What does H+ (i.e. the proton) form in aqueous solution?
A linear hydronium ion [H5O2]+
i.e. H+ in the middle of 2 water molecules
Why is H able to form the hydride anion, H-?
Because H has a high enough electron affinity
Properties of H-
Very strong Lewis base
Can displace OH- from water (showing that H- is a stronger base than OH-)
H-(aq) + H2O —> OH-(aq) + H2(g)
H- can form saline (salt-like) hydrides, and resembles an F- ion in salts e.g. NaF. H- has approx. the same ionic radius as F-
Can form more covalent hydrides with more electronegative metals e.g. [Al2H6]
What is one of the most important properties of the proton?
Its ability to form H-bonds when attached to more electronegative donor atoms e.g. N, O, F
How are H-bonds judged to be present?
When the separation between the 2 atoms forming the H-bond is less than the sum of the vdW radii
i.e. the molecules are closer together than would be expected from vdW forces alone
Typical O-H—O distance
2.76 A
Typical F-H—F distance
2.55 A
Typical N-H—N distance
3.00 A
Group 1
The alkali metals
Very reactive - react violently with water to give the metal hydroxide and H2(g)
e.g. Li(s) + H2O —> Li+(aq) + OH-(aq) + H2(g)
Very negative reduction potentials
Why do group 1 metal ions have a limited ability to form complexes in solution?
Due to their large size and low charge
Metal hydroxides are completely ionised to give the free metal cations and hydroxide ions so are therefore strong bases
What happens to the coordination numbers of the alkali metals going down the group?
Increases due to their increasing size
Li+ CN4-6
Cs+ CN8-9
Why are alkali metals hard in the HSAB sense?
They have low electronegativity
Most important aspect of alkali metal chemistry
Their ability to bind to crown ethers and cryptands
What are crown ethers/cryptands derived from?
Ethylene glycol HOCH2CH2OH
Interior of crown ether cavity
Water-like
Exterior of crown ether
Hydrocarbon-like
18-crown-6
Strong preference for K+ Stability constant (logKa) = 6.08
Important aspect of crown ethers
Can complex alkali metal cations in solution
Provided an explanation for how the selective passage of Na+ and K+ through ion channels in cell membranes may be achieved
Why does 15-crown-5 bind to K+ with a higher stability constant than Na+?
The 15-crown-5 macrocyclic ring is too small for K+ to ‘fit’ inside, so instead 2:1 ‘sandwich’ complex is formed with K+ sandwiched between 2 crown ether rings
Cryptands
3D cavity
Form much more thermodynamically stable complexes with group 1 and 2 metals c.f. crown ethers because it is kinetically difficult for the metal to leave once it is inside the cryptand
Uses of cryptands
Disposable cassettes for measuring blood pH, pCO2, pO2, Na+, K+, Ca2+ etc
OPTI critical care analyses
Group 2
Alkali earth metals
Resemble the alkali metals in most aspects (low electronegativity, hard), but the main difference is their charge (+2), therefore alkali earth metal cations are stronger Lewis acids
Why is the complexing of group 2 cations largely confined to O donors (or N donors if they are part of a ligand also containing O donors e.g. EDTA)?
Because they are hard
Evidence for effect of charge on Lewis acidity of hard metal cations
Study K for the ions Na+, Ca2+, La3+ and Th4+ with EDTA
Increasing K with increasing charge
Do group 2 cations form more or less stable complexes with crown ethers and cryptands than group 1?
More stable
e.g. Ba2+ has a higher stability constant with 18-crown-6 than K+
Same size
Higher charge = stronger binding
Group 3
Higher electronegativity metals
Group 3
Higher electronegativity metals, esp. for Ga, In and Tl (i.e. post-TMs) - these therefore have a very different chemistry to B and Al
Form trivalent cations that form very strong complexes
Tl3+
Tl3+ ion = extremely powerful electron acceptor (Lewis acid)
Stabilised by complexation with ligands
Why is Tl(III) classified as soft?
Due to its high electronegativity
Forms stronger association with Cl- than F-
What is the most stable oxidation state of Tl?
Tl(I) (not Tl(III)!)
Due to the inert pair effect
Inert pair effect
The tendency for the 2 electrons in the outermost s-orbital to remain unionised/unshared in post-TM compounds
The s-electrons are more tightly held to the nucleus due to poor shielding by d- and f-orbitals, so are more difficult to ionise
The inert pair effect refers to the increasing stability of an oxidation state 2 less than the group valency for heavier elements in groups 13-16
Inert pair effect
The tendency for the 2 electrons in the outermost s-orbital to remain unionised/unshared in post-TM compounds
The s-electrons are more tightly held to the nucleus due to poor shielding by d- and f-orbitals, so are more difficult to ionise
The inert pair effect refers to the increasing stability of an oxidation state 2 less than the group valency for heavier elements in groups 13-16
Why does Tl(I) have a tendency towards covalency?
It is soft
Forms complexes where it is bound by soft donors e.g. S
