Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on July 6, 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/.
Lewis and VSEPR structures of Main Group Polyazides
1. Recently, the “highly energetic” tetraazidoborate anion was structurally characterized as part
of the salt 2,2,6,6-tetramethylpiperidinium tetraazidoborate.
(a) Draw the Lewis structure for the uncoordinated azide anion, (N3)–.
The first resonance structure below is the most important contributor for the isolated azide
ion.
-2
-2
N
N
N
N
N
N
N
N
N
(b) Now draw the Lewis structure for the tetraazidoborate anion, [B(N3)4]– and make a
prediction of the 3-D shape of this anion, estimating any bond angles that you can.
N
N
N
N
N
N
B
N
N
B
N
N
N
N
To conserve space, I have only drawn the coordinated azide on one of the 4 nitrogen
positions. The other 3 will look the same as what I have drawn above. The shape around
the boron atom will be tetrahedral, but the bent geometry at each coordinated nitrogen
will cause the overall shape to look like something quite warped from a tetrahedron. The
N–B–N bond angles should all be the ideal 109.5˚. The B–N–N bond angle will be either
something less than 120˚ or something less than 109.5˚, depending on which of the two
resonance structures above is more dominant in the true description of the bonding in the
molecule. The important point is that the azide “arms” will be bent or kinked at the
coordinating nitrogen atom.
2. Silicon tetraazide, Si(N3)4, is reported to be violently explosive! But surprisingly, the salt of
[PPN]2[Si(N3)6] is reasonably stable with a melting point of 225˚C. Of course go a little
higher than that and… BOOM! PPN is a honkin’ big cation with a +1 charge, used to
crystallize the [Si(N3)6]2– anion for crystallographic characterization.
(a) Draw two important resonance structures for the [Si(N3)6]2– anion.
In the molecule below, each of the six “arms” coming off of the silicon atom would look
the same. I have only drawn one example of the bound azide anion.
Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on July 6, 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/.
N
N
N
N
N
N
N
-2
Si
N
N
N
N
N
N
-2
Si
N
N
N
(b) Predict the geometry around the silicon atom and the N–Si–N bond angles.
The electronic geometry and shape around the silicon atom should be octahedral and all
the N–Si–N bond angles should be the ideal 90˚.
(c) Predict a value for the Si–N–N bond angles and the N–N–N bond angles; in other
words, tell me about the geometry of each bound azide group.
The Si–N–N bond angles will be bent, to what degree, depends on the relative importance
of the two resonance structures above. The resonance structure on the left would predict
a trigonal planar geometry around the bound azide N and that that the Si–N–N bond
angles should be less than 120˚. The resonance structure on the right would predict a
tetrahedral geometry around the bound azide N and that the Si–N–N bond angles should
be less than 109˚.
The N–N–N bond angle in either case should be linear and very close to 180˚.
(d) Tell me what you would predict for the N–N bond lengths in the bound azides. Will they
be the same? Will they be different? If so, which one would be shorter?
Since both of the resonance structures above will contribute to some extent, the two N–N
bond lengths should not be the same. The outer N–N bond should be shorter, as it is
something intermediate between a double and triple bond whereas the inner N–N bond
should be slightly longer, as it is something intermediate between a single and double
bond.
And, now the drum roll…for the actual structural parameters? The mean inner N–N
bond length is 1.202 Å and the mean outer N–N bond length is 1.145 Å. The Si–N–N
bond angles are slightly larger than what we expect, about 123˚. The N–N–N bond angle
is almost linear, but bent ever so slightly, about 176˚. To read more, see J. Am. Chem.
Soc. 2002, 124, 12396-12397.