In the following figures are shown some examples of oxygen coordination around hydrogen. They are displayed in a certain order to show, why cut off criteria are more or less arbitrary.
In the first figure we see a typical hydrogen bridge with a primary O-H-bond below 100 pm, a secondary O-H-bond with a length between 170-200 pm and an angle near 180°. All other O-atoms in the neighborhood are traditionally not included in the coordination. This picture is therefore a typical coordination, where an assignment of the coordination number 2 for hydrogen seems out of any discussion.
O3 | --- | |
O4 | 177.7 | |
O1 | 87.4 | |
O1 | 87.4 | |
O3' | 131.1 | |
O3' | 131.1 |
In the next figure we see a bifurcated called hydrogen bond in the same structure. The primary O-H-bond is nearly unchanged. The second and third longest O-H-distances are now nearly the same. The angles are no longer near 180°. The coordination number in this case has to be expanded at least to 3.
O4 | --- | |
O1 | 144.1 | |
O2 | 158.1 | |
O1' | 71.3 | |
O1'' | 92.7 | |
O1'' | 92.7 | |
O2' | 95.8 | |
O2' | 95.8 |
Three symmetrical equivalent second shortest bonds we see in brucite, Mg(OH)2 in the third figure. The distance between hydrogen and the second nearest neighbours are now already near 2.5 Å, a distance often no longer accepted for hydrogen bonds.
Two features make this structure especially interesting.
There is also no qualitative break between the bifurcated and the trifurcated hydrogen bond.
O1 | --- | |
O1' | 131.4 | |
O1' | 131.4 | |
O1' | 131.4 |
And if we go now to LiOH in the last figure we see even four symmetrical equivalent second shortest bonds with O-H-distances already above 3 Å and the only hydrogen-bond with an angle of exactly 180 ° has nearly the double lengths as the typical hydrogen bond in the first example.
O1 | --- | |
O1' | 124.4 | |
O1' | 124.4 | |
O1' | 124.4 | |
O1'' | 180.0 |
We have now followed some examples. We started from a coordination number of 2 to more and more furcated bonds. Continuing in a circle with the first example again, we should be no longer sure to treat it as coordination number 2 and so we may end up with multi-furcated O-H-bonds in all cases.
Some possible solutions to this problem will be presented in the next chapter.
Last Updated: 26 October 1996