November 19, 1947

Dr. R. E. Rundle

Iowa State College

Ames, Iowa

Dear Bob:

I have read your manuscript on "A New Interpretation of Interstitial Compounds" with much interest. It sounds very good to me, and I do not have many comments to make.

As to your suggestion that the two-thirds bonds formed by a light atom involve two sp hybrid orbitals and then the other two p orbitals, it seems to me that this may well be reasonable. I suppose that a two-thirds bond might well be best described from resulting from resonance between an sp bond and a pure p bond. Of course, the extent to which such a detailed description of a resonating structure has meaning is not very clear, and until further useful applications of the general idea have been made we cannot be sure that the idea is worth while.

On page 8, reference 7, the date of the second edition of my book is given as 1942. It should be 1940 - I think that the Cornell University Press may have changed the date line in some of its later printings, but there was no revision whatever after 1940, and 1940 is accordingly the right date.

Would it not be just as easy to combine Tables 1 and 2 into a single table? It would be more convenient for the reader, I think, who, when he referred to the table for a value of an interatomic distance, would immediately see the structure to which it refers.

You may not have noticed that the argument which you give beginning at the bottom of page 15 on the importance of the metal-metal bonds in determining structure is essentially the same argument that I used in my J.A.C.S. paper for explaining the assumption of the nickel arsenide structure by AuSn rather than the sodium chloride structure.

I agree completely with you that often the bond numbers calculated by the straightforward application of my rule to observed interatomic distances are too large in some cases. I have made a big

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collection of crystal structures in which it is necessary to assume that only the strong bonds really have the bond numbers indicated by their distances, the somewhat longer distances being merely incidental to the bond structure. I may mention that in some of these crystals the interatomic distances that actually correspond to bonds are those between like atoms, and not those between unlike atoms, as in the cases discussed by you. Consequently, I think that it cannot be said as a rule the bonds are formed between unlike atoms.

I trust that you will continue with work in this field, and in particular also with experimental studies of these substances.

Sincerely yours,

Linus Pauling:w

P.S. I have gathered together a large amount of material on the interpretation of reported crystal structures, some of which I hope to get published during the coming year. - L.P.