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.
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.
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.
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.
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