无机化学(6.2) Molecular Orbitals (Part 2)

Ionic Bonds
Li 2s donates e- to the F 2pz. In the MO description, these are the 2 orbitals of correct symmetry to interact. The energy difference is > 12 eV. The MO picture looks similar to a covalent interaction
MOs for Larger Molecules – CO2
– Group orbitals from reducible representations
CO as a Ligand Carbon monoxide is known as a σ donor and a π acceptor ligand. It donates electrons from its HOMO to form a sigma bond with the metal.
CO as a Ligand Carbon monoxide accepts electrons from filled d orbitals on the metal into its antibonding (LUMO) orbital.
– – – –
Reducible representations for the outer atom orbitals Group orbitals from reducible representations Match orbitals on the central atom Formation of molecular orbitals
Orbital Potential Energy
Molecules with Polarity - HF
The 2pz orbitals on fluorine interact with the 1s orbital on hydrogen. The px and py orbitals on fluorine lack proper symmetry to interact with hydrogen, and remain as nonbonding orbitals.
MoleculБайду номын сангаасs with Polarity - HF
The anti-bonding orbital resides primarily on the less electronegative atom (H). Note that the subscripts g and u are not used, as the molecule no longer has a center of symmetry.
MOs for Larger Molecules – FHF‒
MOs for Larger Molecules – FHF‒
Orbital H(1s) F(2pz) F(2s)
Energy -13.6eV -18.7eV -40.2eV
H(1s) can’t overlap with GO #1(F2s): right symmetry, wrong energy H(1s) can overlap with GO #3 (F2pz): right symmetry and energy
Carbon Monoxide In carbon monoxide, the bonding orbitals reside more on the oxygen atom, and the antibonding orbitals reside more on the carbon atom.
Carbon Monoxide
LUMO
HOMO
Carbon Monoxide
The highest occupied molecular orbital of CO is a molecular orbital which puts significant electron density on the carbon atom.
Molecular Orbitals from d-Orbitals
Transition metals and other heavy elements use d-orbitals in their bonding interactions
-bond: s/dz2 interaction -bonds: px/dxz & py/dyz interactions
MOs for Larger Molecules – CO2
• • – – The group orbitals for O • O are the same as for F • F The central C has filled s and p orbitals to use in bonding Using D2h character table as a simplification for D∞h point group Coordinate system
Linear Triatomic Anion – FHF‒
[HF2]‒ is the smallest of the linear triatomic anions. [HF2]‒ has the strongest internal hydrogen bonds known. [HF2]‒ forms readily by abstraction of protons: 2 F‒ + RH R‒ + HF + F‒ [HF2]‒ + R‒
Heteronuclear Diatomic Molecules
For heteronuclear diatomic molecules: 1. MO pattern is same as homonuclear diatomics 2. The bonding orbital(s) will reside predominantly on the atom of lower orbital energy (the more electronegative atom). 3. The anti-bonding orbital(s) will reside predominantly on the atom with greater orbital energy (the less electronegative atom).
Carbon Monoxide
The lowest unoccupied molecular orbital of CO is the π* orbitals. The lobes of the LUMO are larger on the carbon atom than on the oxygen atom.
CO as a Ligand
This phenomenon is called back bonding. The increased electron density in the antibonding orbitals of CO causes an increase in the C-O bond length and a decrease in its stretching frequency.
MO Diagram for FHF‒
Central atom on left. 7 F GO’s are non-bonding (lone pairs). GO #3/H(1s) give bonding and antibonding MO’s. Bonding Description: 3 center 2 e- bond
Carbon Monoxide When acting as an electron pair acceptor, the lowest unoccupied molecular orbital (LUMO), is significant.
Carbon Monoxide When acting as an electron pair donor, the highest occupied molecular orbital (HOMO), is significant.
R. O. Ramabhadran, et al. J. Am. Chem. Soc. DOI: 10.1021/ja500125r
MOs for Larger Molecules – FHF‒
D∞h
Consider separately the central atom and its outer atoms Linear = D∞h ~ D2h Character Table for symmetries Group Orbital = SALC (symmetry adapted linear combination) Combine orbitals of outer atoms with same symmetry New group orbitals are then overlapped with central atom AO’s Same combinations as in F2, but separated by a central atom (dot) Each combination produces bonding and antibonding type GO’s
MOs for Larger Molecules – CO2
– Reducible representations for the outer atom orbitals
2s
2pz
For 2s orbitals of the outer oxygen atoms