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I'm here to share with you something that totally confuses me, as I can't see the logic behind it, and my teacher doesn't know either.

Let's take a set of bonds that's trigonal pyramidal, with a lone pair of electrons and three bonds. The angle between the bonds is 107 degrees.

Now, the part where I get confused is if, say, we have one double and two single bonds (I know this makes the atom have 10 electrons, but just ignore that. It's not the point).

Now, in my brain, it would be logical that the double bonds, containing four electrons, would repel more than a single bond, containing just two. However, according to what I've been taught, it doesn't. What is that?

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The way you have learnt to predict molecular geometries is called the VSEPR theory. As all theories, it makes assumptions and has a certain number of limitations. The issue you are asking about is one such limitation.

When predicting the geometry of molecules with VSEPR, one should treat multiple bonds as a single entity. However, the geometry obtained is just a first approximation, or “idealized” geometry. In fact, multiple bonds induce distortions from this ideal geometry, pretty much like lone pairs do (I assume you know that already. If not, well, check the above-linked Wikipedia page for “lone pair”.) Citing from this online course:

Multiple bonds contain a higher electronic-charge density than do single bonds, so multiple bonds also represent larger electron domains ("fatter balloons"). Consider the structure of phosgene, Cl2CO, which is shown below.

enter image description here

From the Lewis structure of phosgene, we might expect a trigonal planar geometry with 120°-bond angles. However, the double bond seems to act much like a nonbonding pair of electrons, reducing the ClCCl bond angle from 120° to 111°. In general, electron domains for multiple bonds exert a greater repulsive force on adjacent electron domains than do single bonds.

So: yes, lone pairs and multiple bonds give rise to deviations from the ideal VSEPR geometries. It's nice that you figured it out on your own!

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  • $\begingroup$ The VSEPR theory I have been taught doesn't even state that different bond types and lone pairs all introduce the same "amount" of repulsion, but that they are indeed different, with the "spatial need" following the trend LP>triple>double>single bonds. I'm a bit confused why any form of theory should assume otherwise ... $\endgroup$ – Antimon Oct 23 '12 at 13:11
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    $\begingroup$ @Antimon well, in order to get the regular patterns as a starting base, you need to say that variations in “repulsion” (or “spatial need”) are a second-order quantity. Otherwise, you cannot get the regular 3D constructions which emerge as first-order approximation of the molecular geometry. $\endgroup$ – F'x Oct 23 '12 at 13:14
  • $\begingroup$ Hmm. I don't see the problem there, since the underlying principles of bonds repelling each other and molecular geometries minimizing those repulsion still remain the same, regardless of whether you assign the same repulsion to them or not. But anyway, in the end, there is no difference between either getting it "right" from the start, or doing it in a two-step process where you apply corrections to a first approximation. $\endgroup$ – Antimon Oct 23 '12 at 13:26
  • $\begingroup$ Thank you, F'x, you just cleared up a lot of stuff in my mind! :) However, I guess I'll just have to stick to 120 degrees because it's what the syllabus teaches (and for simplicity), but it's nice to know what actually happens. $\endgroup$ – DLA Oct 23 '12 at 14:29
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    $\begingroup$ @DLA yes, variation of angles is definitely possible if different atom types are present $\endgroup$ – F'x Oct 23 '12 at 15:15

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