The first electron deficient molecule, diborane

The H3 molecule is an example of a general theorem No nonlinear molecule can have an orbitally degenerate ground state. If calculations indicate that a non-linear molecule would have an orbitally degenerate ground state under a given symmetry, the molecule will in fact distort in such a way that the symmetry is broken. The general statement is referred to as the Jahn-Teller theorem after the scientists who first proved it. 

How large would the distortion be The answer depends on the forces that would tend to oppose the change, i.e. on number of bonding electrons and the nature of the molecular orbitals they occupy. In the special case of the H3 molecule, the two bonding electrons in are not sufficient to prevent the distortion to continue until we are left with a H2 molecule and a H atom. 

The diborane molecule is characterized by three twofold symmetry axes at right angles to each other if we place a Cartesian axis system with the origin at the midpoint of the B-B vector with the x-axis perpendicular to the four-membered ring, the y-axis running through 

Problem 12.5 See Fig. 12.3. Compare the Al-Al the Al-Ct and Al-Ht, bond distances in tetram-ethyldialane and the corresponding bond distances in digallane with the bond distances calculated 

---