W. E. ~ a s e n t
Victorio University of Wellington Wellington, New Zeolond
I
Nan-existent Compounds
There exists a large and diverse group of compounds whose structures do not offend the simpler rules of valence, but which nevertheless are characterized by a high degree of instability; in many cases the compounds in question have never been prepared. The purpose of this review is to collect such compounds together and to inquire into the basis of their instability or non-existence, which in some cases is well understood, in others only imperfectly. I t is considered that textbooks of chemistry might well pay more attention to this aspect of the subject, since inquiry into the reasons for the non-existence of a particular compound can often be an educationally profitable pursuit. Such compounds constitute a healthy supplement to the diet of well-established compounds usually provided for student consumption. For the purposes of discussion, the compounds have been divided into a number of classes; in each of these the low stability of the members can usually he traced to one or more common sonrces, except for the final miscellaneous group. Class 1
Class 1 compounds are those whose instability is a consequence of the restriction of ~t~st-row atoms to a valencr octet. The valence shell of the first row elements can accommodate a maximum of eight electrons in the 2s and 2p orbitals. It is well known that the formation of more than four covalent bonds by these atoms would necessitate the use of their 3d orbitals, and the energy difference between these and the orbitals of the preceding quantum level is so large as to make the promotional energies involved in their use excessively high.
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Journal o f Chemical Educofion
Although Gillespie ( 1 ) has argued that this factor does not completely exclude the possibility of octet expansion, the use of 3d orbitals in stable compounds of first row atoms has never been demonstrated. Some compounds whose existence might otherwise have been expected, with their stable second row analogues in parentheses, are KaaBFo (NarA1F6), R3B(CZ01)3 (K3A1(C20&), Na2CF6 (Sa2SiIpG), KFs (PF6), XaNFe (NaPF6), OF4 (SF \A"*-,.
(26) HOUTEN, S., VOX,A,, AND WREGERS,G. A., Ree. Trav. Chim., 74, 1167 (1955). (27) E. H., A d a Crvsd., . . HOUTEN. . S... AND WIEBENGA, . . 10,. 156 (1957). (28) KUCKEN,W., AND BUCHWALD, H., Angew. Chem., 68, 791 (1956). (29) VAN WAZER,J. R., "Phosphorus and Its Compounds," Interscience Publishers, Inc., New York, 1958, pp. 215714 -A-.
(30) SIDGWICK, N. V.,Ann. Repts. on Progr. Chem. (Chem. Sac. London), 20,120 (1933). (31) . . GRIMM,H. G., A N D SOMMERFELD, . A,,. Z. Phvsik.. . . 36, 36 (1926). R . S., J . Ph,t/s. Chem., 62,353-357 (1958). (32) DRAGO, (33) PAWLING, L.,op. eil., p. 95. (34) PAULING,L., ibid., p. 86. (35) SCHOMAKER, V.,quoted by PAULING, L., ibid., p. 464.
