The semi-empirical EHT method has been applied to diborane B2H6. In agreement with structural data, the EHT method gives the lowest energy (most stable form) to the bridged form. Various computed molecular parameters of diborane as the BH bond distances, various force constants, and the dissociation energy agree with the experimental values. Diborane is compared with ethyl action and the dissimilarities noted. The relevance to the nonclassical carbonium ion problem is discussed. Diborane is bridged because boron seems rather to pσ-bond with the B atom. The coefficients of the wave functions of diborane support this conclusion. Likewise the population of the BB bond drops from 0·30 to 0·02 as the bridged ground state is transformed into the ethane-like form. Ethyl cation, C2H+5, is more stable by EHT theory as an unbridged classical ion with a high barrier to isomerization via the bridged ion to form the rearranged +CH2CH3 ions. The EHT method predicts an activation energy of 23 kcal/mole or greater, a value more than sufficient to prevent CH3CH+2 from becoming +CH2CH3. Ethyl cation is probably classical because the CC pσ-bond is quite strong. Thus the analogy that electron deficient systems of carbon should be bridged because the isoelectronic boron hydrides are bridged, is probably incorrect. © 1968.
