Assignment #4
1. The following is the molecular orbital diagram for the HF2- anion.
H
[F-H-F]-
a) Fill in the electrons for both the F2, H, and the anion
b) Label the MO’s as bonding, non-bonding, or anti-bonding.
c) Draw the MO’s for the combined H and F2 systems
F-F
d) Determine the bond order of the HF2- anion and describe its bonding.
Bond order = (1/2) x (# of bonding electrons - # of non-bonding electrons)
= (1/2) x (2-0) = 1
The bond order for the entire system is one, which means two electrons are being shared
over 3 atoms. This is called a 3 centre – 2 electron bond. We saw this in the dimerization of
borane (BH3) where the bridging H-B-H bond was 3 centre – 2 electron as well.
2. Draw the Lewis structure for NH3 and BF3. Use VSEPR to explain to assign the geometry
and explain the difference between the two compounds.
The geometry of ammonia is trigonal pyramidal (the electron domain assignment is
tetrahedral). The geometry of borane is trigonal planar (so is the electron domain). The
difference here is that the central atom has lone pairs on nitrogen for ammonia but does not
for boron for borane.
3. Assign the hybridization of the central atom in the following cases: H2O, BH3, NH3
O is sp3, B is sp2, N is sp3
4. What are ligand group orbitals (LGO) and how do they simplify molecular orbital theory
for polyatomic molecules?
Ligand group orbitals consider the molecular orbital depiction for peripheral atoms as the
atomic orbital set for the new molecular orbitals. This allows us to make more complicated
molecular orbital diagrams in much the same way as we constructed them for simple
diatomic molecules.
5. Draw the MO diagram for linear XH2,? What is the point group of linear XH2?
See notebook for MO diagram.
The point group assignment is D∞h
6. Draw the MO diagrams for XH2, H2O, BH3, CO2
See notebook or H&S for MO diagrams