TPK

Question 1

Valence bond theory

Answer 1

AO’s in ONE atom mix to form new hybrid orbitals. e.g., s + p + p = sp2.

Question 2

Types of hybrid orbitals from different combinations of s and p.

Answer 2

• sp: tetrahedral (point to the four corners of a tetrahedron)
• sp2: trigonal planar (120deg bond angle)
• sp3: linear (180deg bond angle)

Question 3

Molecular orbital theory (MO theory)

Answer 3

The AO’s from TWO atoms overlap to form TWO new MO’s. The lower energy is referred to as the HOMO and the higher energy the LUMO.

Question 4

Key points to note about MO theory

Answer 4

1. Number of MO’s = number of combining AO’s.
2. Bonding MO’s are lower energy than parent AO’s and anti-bonding MO’s higher energy.
3. Electrons in molecules are assigned to orbitals from lowest to successively higher energy.
4. AO’s combine most effectively when they are of similar energy.
5. The closer an MO is in energy to an AO, the more its character is like that AO.
6. MO’s with more nodes are generally higher energy.
7. LUMO’s have significant roles to play in reaction mechanisms.

Question 5

Implications of a large energy difference between two combining AO’s

Answer 5

• This implies a large electronegativity difference between combining AO’s.
• Implies the bond is strongly polarised.
• Will form weaker bonds, with greater ionic character.
• Susceptible to nuc- attack at M or elec+ attack at C.

Question 6

Implications of a small energy difference between two combining AO’s

Answer 6

• This implies a small electronegativity difference between combining AO’s.
• Implies the bond is weakly polarised.
• Will form strong bonds, with greater covalent character.
• Only reacts with reactive species, e.g., radicals.

Question 7

Reactivity of organometallics with oxygen

Answer 7

• Organometallic compounds are readily oxidised by oxygen.
• This gives MOn + H2O + CO2.
• Organometallic compounds can be labile against O2 and H2O when they possess: (I) free e- pairs, (II) low-lying empty orbitals, (III) highly polarised M-C bonds.

Question 8

Compare FMO’s for polar and non-polar organometallics and their subsequent reactivity

Answer 8

Polar: large electronegativity difference between combining M and C AO’s; M(delta+) is susceptible to nuc(-) attack; C(delta-) is susceptible to elec(+) attack.
Non-polar: small electronegativity difference between combining M and C AO’s: reacts with radicals.

Question 9

Bonding in Alkyl-Li compounds

Answer 9

• 4C-2e bond (one MO extended over four different atoms).
• 3 (p) AO’s from 3 Li atoms interact with 1 (sp) AO from 1 methy group.
• Each Li atom donates 1/3 e- and each methyl group 1 e- to the bond.
• This bonding is less stable and more reactive than standard covalent bonding.

Question 10

3C-2e bonding

Answer 10

• e.g., Grignard reagent.
• Less reactive than 4C-2e, but still more reactive than standard covalent.
• Model uses sp3-hybridised MO’s on Mg interacting with C orbitals.
• This model is symmetrical, allowing us to view half number of interacting orbitals (3 instead of 6!)

Question 11

MLX model

Answer 11

• M=metal, L=2e ligand, X=1e ligand.
• Total valence count for compound = M group number + X + 2L - charge.
• M oxidation number = X - charge.