topic 4 - MO theory - heteronuclear diatomics + ligand exchange

HOMO

• the highest occupied molecular orbital

• contains the highest energy hence most nucleophilic electrons

LUMO

• the lowest unoccupied molecular orbital

• the place where electrons will be added to the molecule ie. where a nucleophile would attach

SOMO

• a singly (ie half filled) molecular orbital

paramagnetic

• at least one unpaired electron

• attracted into an induced magnetic field due to the presence of unpaired electrons

Diamagnetic

• all electrons are paired

• repelled by an induced magnetic field due to the presence on only paired electrons

sigma-pi crossover

• the inversion of energy of the sigma and pi orbitals is due to the change in energy of the s and p orbitals that form them

• when the s and p orbitals are of similar energy then extra mixing can occur if the symmetry is also correct

• energy levels between s+p are much closer in B,C+N than they are in O etc. allows orbitals to switch in energy when MOs are formed

• the crossover is between N and O

• The interaction between the sigma (σ) 2s and sigma (σ) 2p orbitals pushes the energy of the sigma (σ) 2p orbital up and can even cause it to become higher in energy than the pi (π) 2p orbitals. 

what is the basis set?

• all of the atomic orbitals present in the bonding atoms that are used to form MOs

how to determine what energy a bond formed has?

• typically the most electronegative atom contributes to the bonding and least contributes to anti-bonding

• eg. Li-H bond - has hydrogen character associated bc H1s is lower energy so closer to bonding orbitals

how to determine which molecule is most unstable

• the one with the most electrons in anti-bonding MOs

LGO approach for polyatomic molecules

• use VSEPR to determine the molecular shape

• define linear combination of ligand AOs

• add central atom electrons and think about MOs

• go through each possible LGO with side atoms being both same phase or different phases and combining with each of the possible orbitals from the central atom excluding those that wouldn’t have sufficient overlap eg. 2px and 2py wouldn’t overlap with a 1s but 2pz could bc its in the same plane

phases of atomic orbital

• no difference in terms of probability of finding an electron

• different shading refering to the sign of the wavefunction

non-bonding MO energy

• no change in energy from AO’s bc the in-phase and out of phase overlap cancel out