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One Shocked Chemist

Molecular surprises are sometimes right in front of us, if only we’d do the math

Roald Hoffmann

This is the story of a theoretical result that astonished me. But it shouldn’t have.

A chemist for nigh on 50 years, I’ve had the joy of seeing lots of new molecules in the literature. Let me show you a couple, just so you see what surprises me.

2011-03MargHoffmannFA.jpgClick to Enlarge ImageSome new molecules are striking, just because they are extensions of familiar motifs that no one previously thought of changing. Take the “N-confused” porphyrins, compounds with a fitting name that tells us something about our history, expectations, and the nice informality of chemical nomenclature. Porphyrins (first figure, top) are a classic group of molecules in which four so-called pyrrole rings—five-membered rings with four carbons and a nitrogen—face their nitrogens toward an interior void that is often neatly filled by a metal atom, for instance an iron in the heme unit of hemoglobin. In the N-confused porphyrins (first figure, bottom), one pyrrole ring is turned, and a carbon atom faces the center, not a nitrogen atom. It makes a difference, of course. And I’m glad someone thought of ringing the changes on a classic motif. N-confused porphyrins were first reported by two groups in 1994, those of Lechoslaw Latos-Grazynski at the University of Wroclaw in Poland, and of Hiroyuki Furuta of Oita University in Japan.

Some new molecules are simply astounding. We have learned that xenon, a so-called noble gas, is far from inert, forming bonds with halogens, oxygen and carbon. But I never imagined a bond between two relatively unreactive elements, gold (Au) and xenon (Xe), and, to boot, with a pretty naked xenon acting as a ligand—which is usually an ion or a molecule that binds to a central metal atom by donating one of its electron pairs. But that’s what Stefan Seidel and Konrad Seppelt of the Free University of Berlin made in 2000 in the square-planar AuXe42+ ion shown in the second figure.

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