Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on July 13, 2009. Copyright Margret J. Geselbracht 2009. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
Boron-boron Multiple Bonds: Lewis Structures and Molecular Orbitals
These are two exam questions that were put on sequential in-class exams. Each exam was
worth 100 points total, and the relative points for each of these questions are shown. The blue
and white molecular orbital surfaces showed sufficient contrast when photocopied.
1. (10 pts.) The chemistry of boron-boron multiple bonds is quite rare. About a year ago, the
novel diboronyl diborene dianion with a linear [OBBBBO]2– structure was observed
experimentally in the gas phase. (a) Draw two resonance structures for this molecule,
clearly labeling any formal charges.
O
B
B
B
B
O
O
B
B
B
O
B
B
O
B
B
B
O
(b) Computational methods established the central B–B bond was significantly shorter
(1.455 Å) than the outer B–B bonds (1.605 Å). With which of your two resonance
structures is this data consistent? Are you surprised?
The short central B–B bond is consistent with the top right structure with a triple bond
between the B atoms. This is surprising since boron carries a negative formal charge
and yet it is less electronegative than oxygen.
2. (18 pts.) Remember the diboronyl diborene molecule with a linear [OBBBBO]2– structure
from the last exam? Well, it’s back! Pictures of 11 of the 13 occupied molecular orbitals are
shown below.
πu
O B
O B
B
B
B
B
B O
u
B O
g
B O
u
B O
HOMO-7
HOMO (one of degenerate set of 2 MOs)
g
O B
B
B
B O
O B
O B
B
B
B
HOMO-8
HOMO-2
πg
B
B O
HOMO-3 (one of degenerate set of 2 MOs)
O B
B
B
HOMO-9
Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on July 13, 2009. Copyright Margret J. Geselbracht 2009. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
πu
O B
B
B
B O
O B
B
B
B O
g
HOMO-10
HOMO-5 (one of degenerate set of 2 MOs)
(a) Label each of the MOs above as - or π-type symmetry and gerade or ungerade symmetry.
(b) Calculations on [OBBBBO]0/1/2– in the neutral, anionic and dianionic states show the same
set of degenerate orbitals for the HOMO in each case, containing different numbers of
electrons. In these molecules, the B–O bond lengths increase slightly from the neutral
molecule to the dianion, whereas all the B–B bond lengths decrease slightly. Explain these
observations.
The HOMO has boron-boron π-bonding character but boron-oxygen π-antibonding
character. So, putting more electron density in the HOMO should weaken and lengthen the
B–O bond but strengthen and shorten the B–B bond.
(c) Draw cartoons for HOMO-2 and HOMO-10 showing how the atomic orbitals on B and O
overlap to form these two molecular orbitals.
HOMO-2
O B
B
B
HOMO-10
B O
O B
B
B
B O