1 October 1963
Professor F. A. Cotton
Chemistry Department
Massachusetts Institute of Technology
Cambridge, Massachusetts
Professor G. Wilkinson
Imperial College of Science and Technology
London, England
Dear Professors Cotton and Wilkinson:
I have just been reading some parts of your interesting textbook on Advanced Inorganic Chemistry, and I am writing to comment on some of your statements, with the hope that you might wish to take my comments into consideration when you prepare your revised edition.
I feel that parts of your discussion of my work are improper. You have, of course, the right to present the subject in the way that seems best to you, and particular, to include whatever amount of discussion of my work that seems to you to be justified. Some of your statements, however, seem to me to be of such a nature as to give a student reading the book a false impression.
For example, in your brief discussion of the theory of resonance you mention that resonance structures are only figments of our imagination, that the structures are hypothetical ones, that one should not make the error of attributing physical reality to them, and that you will discuss alternative and perhaps to some tastes more realistic methods later.
I have read several sections of the book in which you deal with the molecular orbital theory, without finding any corresponding statement about this theory – that is, the statement that molecular orbitals do not have physical reality (except for the trivial case of one-electron atom or molecule) but instead are, like the carbon-carbon double bond and other elements of ordinary valence-bond theory and like the individual structures used in the theory of resonance, figments of our imagination which we use to obtain a more or less satisfactory description of a molecule.
Do you see any justification for emphasizing this aspect of resonance theory, and ignoring the same aspect of the classical structure theory of chemistry and of the molecular orbital theory?
As a second example, I may mention your brief discussion, beginning on page 606, of the valence-bond method of discussion complexes of transition elements. This discussion is interesting to me because it is essentially polemic in nature.
First, the brief discussion (less than two pages) of the theory itself is inaccurate and misleading. In the discussion on page 608 you ignore the statements on page 108 of the first edition of my book, and corresponding pages of later editions: “As many as four rather weak covalent bonds could be formed with the use of the 4s and 4p orbitals without disturbing the 3d shell, and a corresponding amount of covalent character of the bonds would not change the magnetic moment of the complex.” A footnote to the year 1957 about the use of 4d orbitals is given, which you refer to the early 1950’s. You have also ignored the discussion on page 38 of my book, relating to the change from “essentially covalent” ground state to an “essentially ionic” ground state, which you refer to in relation to complexes of cobalt at the top of page 608 and complexes of nickel in the middle of page 608.
This sort of misrepresentation of the theory probably will lead a student reading your book to accept, without investigation, your uncomplimentary references to it: “its ad hoc and artificial prescriptions as to the nature of the bonds,” “various other grounds for regarding the VB treatment of transition metal complexes as unsatisfactory,” its predictive value…is often poor and even seriously misleading,” “it is now recognized that this so-called ‘magnetic criterion of stereochemistry is unreliable because a square nickel (II) complex can be paramagnetic, as shown earlier (page 571), and so on.
I am dissatisfied with this section of your book for several reasons. First, your use of pejoratives seems to me to be out of place in a scientific book. Second, you make no mention of the successes of the theory, such as the prediction (1931), later verified, that the diamagnetic complexes of bipositive nickel would be found to have a square planar structure. Third, your presentation of the theory is incorrect or misleading in several respects, one of which is quoted above.
I note that on page 609 you say that the magnetic criterion of stereochemistry is unreliable because a square nickel (II) complex can be paramagnetic, as shown earlier (571). On reference to page 571 I find only a theoretical discussion which I think may be described in your own words as involving the use of some fairly sophisticated quantum mechanics, not such that an experimental inorganic chemist would carry out for himself. My feeling is that the arguments on page 571 is not one that I would accept as justification for your statement on page 609 – I might well prefer to rely on the valence-bond theory, and, and perhaps some of the readers of your book would also, if they were not so prejudiced by your statements as to be prevented from learning about the theory.
I would, of course, accept experimental evidence bearing on this question and I notice that you have a discussion of complexes of bipositive nickel in a section beginning on page 738. I have read this section, without finding reference to any square planar complex of bipositive nickel with magnetic properties incompatible with those expected from the valence-bond theory of complexes.
I conclude that the only basis for your statement that the magnetic criterion is unreliable because a square nickel (II) complex can be paramagnetic is that some complicated theoretical treatment has suggested to you that a hypothetical square nickel (II) complex might be paramagnetic and I object to your having made your statement on this basis, without baring made it clear to the reader.
I doubt that the statements in the last paragraph on page 609 are correct. I have discussed this matter in a paper published in Volume 39, September 1962, of the Journal of Chemical Education.
I enclose a list of some minor errors that I have noticed.
Yours truly,
Linus Pauling:hpg
Encl.
Page 13 The curve is not correctly plotted.
Page 48 Figure upside down
Page 55 I think that the theory of the three-electron bond is
part of valence-bond theory, and that your discussion of the valence-bond theory in relation to molecular oxygen is incorrect.
Page 175 This drawing is extremely poor. The four beryllium atoms can hardly be said to lie at the corners of a regular tetrahedron around the oxygen atom, in this drawing.
Page 199 Many of the hydrogen atoms are indicated in the wrong positions, in the drawings of boranes.
October
1, 1963
