Lecture 7 Flashcards
How do we check that Mo theory is correct?
Photoelectron spectroscopy
What does photoelectron spec gives us?
- energy of orbitals
- identity of orbitals
What is orbital mixing/ sigma-pi crossover?
- Atomic orbitals of the same symmetry and energy can mix to form MOs. MOs of the
same symmetry that are close in energy can also mix. - MOs of same symmetry that are close in energy can also mix.
- When two orbitals mix, one of the new orbitals moves up in E, while the other moves down .
For which elements will the orbitals that have the same symmetry be close in energy?
Elements where the 2s and 2p gap in energy s close together (B,C,N)
Which molecules show sigma-pi crossover?
(Li2, Be2), B2, C2, N2
Why do some molecules show sigma-pi crossover while others don’t?
- due to differing size of the 2s- 2p energy gap
- For O2 and F2, orbital mixing does occur but not enough to make crossover happen
Draw a diagram to show the interaction (mixing) of α2s and α2pz orbitals
page 57
Draw a diagram to show the interaction (mixing) of α2s and α2pz orbitals
page 57
Draw a MO diagram for N2 to demonstrate mixing interactions
page 57
Which orbitals are influenced by s-p mixing?
α2s and α2pz
(not α2s and α2pz)
No pi orbitals are influenced as they are the wrong shape
Give the order of orbitals in O2 and F2 and other diatomics
- O2 and F2 have “typical” ordering of MOs
α pz below π px,py - Other second row diatomics B2 → N2, have inverted ordering
π px,py below α pz
What do MO diagrams tell us?
- Why some molecules exist and not others
- How strong bonds are
- Whether a molecule is paramagnetic
- What electronic transitions can take place
Give the key features of CO
- oxygen AOs are lower in E than carbon AOs. O has a higher Z
- s/p mixing is present
- 2s-2p energy gap in oxygen is bigger than in carbon
State some chemical properties of CO
- Toxicity
- Ligand binding properties
Describe the chemical properties of CO
- Toxicity
Lewis base – it is toxic due to its ability to bind to the iron centre in haemoglobin more
effectively than O2. - Ligand Binding Properties, e.g. Cr(CO)6
- HOMO is C centred lone pair. CO acts as a donor ligand in complexes with low oxidation
state transition metals.
- Its LUMOs are a C centred p* orbital. Accepts electrons from transition metal ligands.
Describe the relationship between bond enthalpy and bond order
as bond enthalpy increases, bond order increases
Describe the relationship between bond distance and bond order
bond distance decreases as bond order increases
Describe the relationship between bond enthalpy and bond distance
bond enthalpy decreases as bond distance increases
Describe the relationship between bond stretching frequency and bond order
bond stretching frequency increases as bond order increases
stronger bonds- harder to stretch
Define isoelectric
2 species are defined as isoelectric if they have the same number of electrons
e.g. CO and N2
They often display the same properties
Define paramagnetic
- molecule with unpaired electrons
- It is drawn into the magnetic field
Define diamagnetic
- molecule with all electrons paired
- pushed out of a magnetic field
