Cation receptors

Question 1

Why is H+ a very strong Lewis acid?

Answer 1

Very small size so has a high charge density

Question 2

H+ in HSAB sense

Answer 2

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-

Question 3

What does H+ (i.e. the proton) form in aqueous solution?

Answer 3

A linear hydronium ion [H5O2]+

i.e. H+ in the middle of 2 water molecules

Question 4

Why is H able to form the hydride anion, H-?

Answer 4

Because H has a high enough electron affinity

Question 5

Properties of H-

Answer 5

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]

Question 6

What is one of the most important properties of the proton?

Answer 6

Its ability to form H-bonds when attached to more electronegative donor atoms e.g. N, O, F

Question 7

How are H-bonds judged to be present?

Answer 7

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

Question 8

Typical O-H—O distance

Answer 8

2.76 A

Question 9

Typical F-H—F distance

Answer 9

2.55 A

Question 10

Typical N-H—N distance

Answer 10

3.00 A

Question 11

Group 1

Answer 11

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

Question 12

Why do group 1 metal ions have a limited ability to form complexes in solution?

Answer 12

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

Question 13

What happens to the coordination numbers of the alkali metals going down the group?

Answer 13

Increases due to their increasing size
Li+ CN4-6
Cs+ CN8-9

Question 14

Why are alkali metals hard in the HSAB sense?

Answer 14

They have low electronegativity

Question 15

Most important aspect of alkali metal chemistry

Answer 15

Their ability to bind to crown ethers and cryptands

Question 16

What are crown ethers/cryptands derived from?

Answer 16

Ethylene glycol HOCH2CH2OH

Question 17

Interior of crown ether cavity

Answer 17

Water-like

Question 18

Exterior of crown ether

Answer 18

Hydrocarbon-like

Question 19

18-crown-6

Answer 19

Strong preference for K+
Stability constant (logKa) = 6.08

Question 20

Important aspect of crown ethers

Answer 20

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

Question 21

Why does 15-crown-5 bind to K+ with a higher stability constant than Na+?

Answer 21

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

Question 22

Cryptands

Answer 22

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

Question 23

Uses of cryptands

Answer 23

Disposable cassettes for measuring blood pH, pCO2, pO2, Na+, K+, Ca2+ etc
OPTI critical care analyses

Question 24

Group 2

Answer 24

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

Question 25

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)?

Answer 25

Because they are hard

Question 26

Evidence for effect of charge on Lewis acidity of hard metal cations

Answer 26

Study K for the ions Na+, Ca2+, La3+ and Th4+ with EDTA

Increasing K with increasing charge

Question 27

Do group 2 cations form more or less stable complexes with crown ethers and cryptands than group 1?

Answer 27

More stable
e.g. Ba2+ has a higher stability constant with 18-crown-6 than K+
Same size
Higher charge = stronger binding

Question 28

Group 3

Answer 28

Higher electronegativity metals

Question 29

Group 3

Answer 29

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

Question 30

Tl3+

Answer 30

Tl3+ ion = extremely powerful electron acceptor (Lewis acid)

Stabilised by complexation with ligands

Question 31

Why is Tl(III) classified as soft?

Answer 31

Due to its high electronegativity

Forms stronger association with Cl- than F-

Question 32

What is the most stable oxidation state of Tl?

Answer 32

Tl(I) (not Tl(III)!)

Due to the inert pair effect

Question 33

Inert pair effect

Answer 33

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

Question 34

Inert pair effect

Answer 34

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

Question 35

Why does Tl(I) have a tendency towards covalency?

Answer 35

It is soft

Forms complexes where it is bound by soft donors e.g. S