June 26, 1967
Professor Otto Bastiansen
Department of Chemistry
University of Oslo
Blindern, Norway
Dear Otto:
I am writing about a paper on the molecular structure of beryllium borohydride, by A. Almenningen, G. Gundersen, and A. Haaland in Chemical Communications, I967, page 557. It seems to me likely that the structure that they suggest is essentially correct, but that the assignment of interatomic distances is incorrect.
Using the nomenclature of their Figure 1, II, I would say that the bond length 1.31Å is to be assigned to beryllium-hydrogen 4, rather than to boron-hydrogen 3. This distance is just about right for beryllium, but too short for the boron bond to a hydrogen atom shared between two borons, to say nothing of the beryllium atom.
I would expect hydrogen atom 3 to be about 1.5Å from boron, and a larger distance from beryllium. The bonds from the beryllium atom to the four hydrogen atoms 2 and 3 should all be very weak, and the distances accordingly long.
The beryllium-boron bond, given as 1.839 + 0.005Å, seems to me to be too short. I would expect a somewhat larger value, 1.95 to 2.00. Also, I would anticipate that the molecule would come closer to showing a three-fold axis, and that the hydrogen atoms 4 should lie in the plane of the molecule.
Altogether, the feeling that I have is that the structure proposed by the authors is not an acceptable one. Perhaps an important structural feature is the looseness of the bonding of BeH2 to B2H6. It may well be that the hydrogen atoms 4 tend to be twisted out of the plane to some extent, one lying above and one lying below the plane, but not, as in the figure, twisted 90º out of the plane.
With best regards, I am
Sincerely,
Linus Pauling
LP: fkg
June
26, 1967
