Tight Squeeze

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Science  02 Apr 2010:
Vol. 328, Issue 5974, pp. 18
DOI: 10.1126/science.328.5974.18-c
CREDIT: DALY ET AL., ANGEW. CHEM. INT. ED. 49, 10.1002/ANIE.200905797 (2010)

One of the first things students learn in a chemistry course is the octet rule: Atoms of the light elements are capable of sharing at most eight electrons, which limits them to four bonding partners (at two electrons to a bond). Of course, rules are made to be broken, as students soon realize when they are confronted with the stability of PF5 and SF6. And then there are the metals, such as iron and chromium, that routinely form six-coordinate compounds by using their partially vacant d orbitals. Notable exceptions in this realm involve coordination by seven guests, but it all depends on how big the guests are; smaller ones such as hydrogen can crowd together more effectively.

This brings us to tetrakis(N,N-dimethylaminodiboranato)thorium. Among the heaviest metals, thorium boasts a comparatively large radius, and Daly et al. have managed to squeeze 15 different hydrogen atoms around it, tethering the central metal to eight boron centers in the periphery. The authors characterized the compound by neutron as well as x-ray diffraction (the former being an especially sensitive technique for hydrogen detection), and their theoretical simulations suggest that, were it not for crystal packing effects, yet another hydrogen would coordinate. Heating this crowded compound induces some chemical rearrangements, the net effect of which is to bring the coordination number down to the rather less shocking value of 14.

Angew. Chem. Int. Ed. 49, 10.1002/anie.200905797 (2010).

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