May 14, 1940

Dr. L. Michaelis

The Rockefeller Institute for Medical Research

66th Street and York Avenue

New York, New York

Dear Dr. Michaelis:

Mr. J. Waser is working with me on the structure of substances such as arsenobenzene, for which the formula customarily written is φ -As=As- φ. We think it probable that this molecule and similar molecules contain no arsenic-arsenic double bonds, but instead have a ring structure such as

[Hand drawn structure]

It has occurred to us that you might have made a determination of the oxidation reduction potential of some such substance or that you might know whether or not someone has made such a study. Can you tell me if this has been done?

When I saw you I forgot to mention that I had Miss French of Wellesley Collage, who was in Pasadena last year on Sabbatical leave, make some magnetic measurements on the iron pentacyano complex which you had studied potentiometrically. This investigation is not yet completed, but the results obtained so far indicate that the structure of the complex is

[Hand drawn structure]

The two iron atoms surrounded by six carbon atoms are more easily reduced than the two surrounded only by four carbon atoms and two nitrogen atoms,

[page 2]

and the magnetic properties of two kinds of iron atoms are different. With this formula there is no water of constitution in the complex.

With best regards, I am

Sincerely yours,

Linus Pauling

LP:jr