| MadSci Network: Chemistry |
Yes, the theory is still widely accepted. The reason it doesn't show up in your high school chemistry book is firstly because it is a very rare type of bonding, which really doesn't show up except in these specific boron compounds and a few unusual metal compounds, and secondly because the editors figure students usually have quite enough trouble understanding chemical bonding without laying this extra complication on them.
It shows good chemical thinking to try to draw an analogy between B2H6 (which you might expect to be BH3 ) and Al2Cl6 forming when AlCl3 dimerizes. But the type of bonding is really quite different.
The common factor in the bonding is the desirability of the group 3 element getting 8 electrons into its valence shell, rather than just the 6 that would arise from 3 covalent bonds. The important difference is that in the boron hydrides, there is a general shortage of valence electrons, so that two pairs are each shared three ways to form a bond between a single hydrogen atom and two boron atoms.
But with aluminium* chloride, there are plenty of lone pair electrons on chlorine not otherwise involved in bonding. One of these is simply shared with the aluminium atom to complete its 8 electron valence shell. In this way two AlCl3 molecules get joined together by two new co-ordinate Al<-Cl bonds between the aluminium of each molecule and one of the chlorines from the other molecule.
Bonding in boranes is quite a difficult subject. If you can get hold of a book by N.N. Greenwood & A. Earnshaw - "Chemistry of the Elements", there is a detailed discussion that is accurate and up to date, but might be a bit difficult for you to follow. (pp. 179-185 in the 1989 edition).
Hope this helps,
John.
* By the way, the International Union of Pure and Applied Chemistry has decreed that "aluminium" rather than "aluminum" is the appropriate name for element 13.