Some coordinations around hydrogen

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.

A typical hydrogen bond

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.

Atom
R in Å
Angle
O3
0.963
---
O4
1.870
177.7
O1
2.913
87.4
O1
2.913
87.4
O3'
3.191
131.1
O3'
3.191
131.1

first picture

A typical hydrogen bond in Lithiumnitrate-Trihydrate, LiNO3(H2O)3 [Hermansson, Thomas, Olovsson 1980]

A bifurcated bond

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.

Atom
R in Å
Angle
O4
0.965
---
O1
2.199
144.1
O2
2.235
158.1
O1'
3.271
71.3
O1''
3.419
92.7
O1''
3.419
92.7
O2'
3.877
95.8
O2'
3.877
95.8

second picture

A typical hydrogen bond in Lithiumnitrate-Trihydrate, LiNO3(H2O)3 [Hermansson, Thomas, Olovsson 1980]

Three symmetrical bonds

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.

Atom
R in Å
Angle
O1
0.995
---
O1'
2.458
131.4
O1'
2.458
131.4
O1'
2.458
131.4

third picture

Coordination in brucite, Mg(OH)2 with 3 symmetrical equivalent second shortest distances to oxygen [Zigan, Rothbauer 1967]

Even higher coordination

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.

Atom
R in Å
Angle
O1
0.937
---
O1'
3.041
124.4
O1'
3.041
124.4
O1'
3.041
124.4
O1''
3.397
180.0

last picture

Coordination in lithium hydroxide, Li OH with 4 symmetrical equivalent second shortest distances to oxygen [Mair 1978]

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.

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Last Updated: 26 October 1996