BORON HYDRIDES BY MAURICE I,. HUGGINS’
According to Stock and his coworkersJ2 there are a t least ten boron hydrides, of which the formulae of four are known: B2H6, B4H10,BcH12, and B10H14. Stock considers that in these compounds boron is quadrivalent, but, in the light of present day evidence in regard to atomic structure, it does not seem possible that boron can be quadrivalent in the sense that carbon is quadrivalent-that in these substances each boron atom can be directly bonded, throughelectron pairs, to four other atoms. Also, the compound BloHleis not simply accounted for in this way; and furthermore, why is it that boron exhibits such a valence only in its hydrogen compounds? Because of the above considerations, the author has been led to look for some other type of structure for these compounds, based on the idea that boron possesses but three valence electrons. The structures arrived at are illustrated in Figures 1 to 4. cs The analogy between these structures and those of the carbon compounds, ethylene (H2C = CH2),butadiene (H2C = CH - CH = CH2), benzene3 (Fig 5 ) , and naphthalene (Pig. 6), is very striking. It has been pointed out by the author.’ that the “residiral affinity” supposedly possessed by “unsaturated atoms” in many organic molecules is more properly considered a property of the double or triple bond, and results in the attraction of other unsaturated structiires and the formation (usually only DuPont Fellow in Chemistry at the University of California, 1921-1922. Stock and Massenez, Ber. deutsch. chern. Ges., 45, 3539 (1912); Stock and Friederici, Ibid., 46, 1959 (1913); Stock, Friederici and Priess, Ibid., 46, 3353 (1913); Stock, Zeit. Elektrochemie, 19, 779 (1913); Stock, Kuss and Preiss, Ber. deutsch. chem. Ges., 47, 3115 (1914). * Concerning the benzene and naphthalene structures, cf. Huggins, Jour. Am. Chem. SOC.,44, 1607 (1922); Science, 55,678 (1922). Jour. Am. Chem. SOC.,44, 1607 (1922).
Maurice L. Huggins
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temporarily) of a bond of three or four (or sometimesmore) electrons. Now if we suppose that boron atoms have a greater tendency to form and maintain such complexes than have carbon atoms, the structures illustrated are accounted for.
“aH H H
H
H H
H H
FIG.1- 8zH6
H H
H H H
fIG.3 - R ~ H I ~
FIG.6- C,oH8
Altho the “residual affinity” of the boron-boron double bond is strong enough to hold onto hydrogen atoms, it may be incapable of holding onto larger atomic kernels. Although the figures picture the hydrogens as in the same plane as the boron nuclei and the bond electrons, it is more
Boron Hydrides
835
probable that the electrons in each four-electron bond and the atomic kernels around each such bond are tetrahedrally ar(and perranged. I t is also possible that in B6H12and BICH14 haps in the others) the boron nuclei are alternately to one side or the other of the plain containing the electronbonds (the plane of the paper, in the figures), the three valence electrons forming, with the two electrons in each boron kernel, a very much distorted tetrahedron around each nucleus. Little is known of the properties of these substances which would serve as evidence for or against this theory. Further experimental evidence is needed before these or any other proposed structures may be considered proved.
Summary 1. Structures are proposed for the compounds B2Hs,B4HI0, B6H12, and BI0Hl4. 2. These structures are analogous to those of the carbon compounds ethylene, butadiene, benzene, and naphthalene, respectively. 3. The hydrogen atoms are held by means of four-electron bonds, such as often exist (temporarily a t least) in unsaturated organic compounds. Each such bond is surrounded by four atoms. Chemical Laboratory University of Califorwia Berkeley, California
