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Hi O Silver

Roald Hoffmann

Incredible, But True

Recently, a young Polish postdoctoral associate, Wojciech Grochala, led me to look with him at the chemical and theoretical design of novel high-temperature superconductors. We focused on silver (Ag) fluorides (F) with silver in oxidation states II and III. The reasoning that led us there is described in our forthcoming paper. For now let me tell you about some chemistry that I learned in the process. I can only characterize this chemistry as incredible but true. (Some will say that I should have known about it, since it was hardly hidden, but the fact is I didn't.)

Here is what Ag(II), unique to fluorides, can do. In anhydrous HF solutions it oxidizes Xe to Xe(II), generates C6F6+ salts from perfluorobenzene, takes perfluoropropylene to perfluoropropane, and liberates IrF6 from its stable anion. These reactions may seem abstruse to a nonchemist, but believe me, it's not easy to find a reagent that would accomplish them.

Ag(III) is an even stronger oxidizing agent. It oxidizes MF6 (where M=Pt or Ru) to MF6. Here is what Neil Bartlett at the University of California at Berkeley writes of one reaction: "Samples of AgF3 reacted incandescently with metal surfaces when frictional heat from scratching or grinding of the AgF3 occurred."

Ag(II), Ag(III) and F are all about equally hungry for electrons. Throw them one, and it's not at all a sure thing that the electron will wind up on the fluorine to produce fluoride (F). It may go to the silver instead, in which case you may get some F2 from the recombination of F atoms.

Not that everyone can (or wants to) do chemistry in anhydrous HF, with F2 as a reagent or being produced as well. In a recent microreview, Thomas O'Donnell says (with some understatement), "... this solvent may seem to be an unlikely choice for a model solvent system, given its reactivity towards the usual materials of construction of scientific equipment." (And its reactivity with the "materials of construction" of human beings working with that equipment!) But, O'Donnell goes on to say, "... with the availability of spectroscopic and electrochemical equipment constructed from fluorocarbons such as Teflon and Kel-F, synthetic sapphire and platinum, manipulation of and physicochemical investigation of HF solutions in closed systems is now reasonably straightforward."

For this we must thank the pioneers in the field—generations of fluorine chemists, but especially Bartlett and Boris Zemva of the University of Ljubljana. Bartlett reports the oxidation of AgF2 to AgF4 (as KAgF4) using photochemical irradiation of F2 in anhydrous HF (made less acidic by adding KF to the HF). And Zemva used Kr2+ (in KrF2) to react with AgF2 in anhydrous HF in the presence of XeF6 to make XeF5+AgF4. What a startling list of reagents!

To appreciate the difficulty and the inspiration of this chemistry, one must look at the original papers, or at the informal letters of the few who have tried it. You can find some of Neil Bartlett's commentary in the article that Wojciech and I wrote, and in an interview with him.

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