The diboron molecule

No dihelium molecule has been detected in the gas phase at room temperature the potential energy curve of a pair of He atoms is in fact strongly repulsive for R 140 pm. (As we shall see below there is an extremely weak attraction between the atoms at larger distances.) The non-existence of the He2 molecule is commonly explained by assuming that two electrons in the antibonding MO formed by combination of the two AOs 

B2 has been detected in high-temperature vapours. The dissociation energy is Do = 290 10 kJ mol and the bond distance Re = 159.0 pm. The bond is thus about three times stronger and 40% shorter than in Li2. WhileLi2 and Be2 are closed shell species, i.e. contain no unpaired electrons, B2 molecules in the electronic ground state have two electrons with parallel spins. This observation suggests that the two electrons occupy two degenerateMOs. 

Spectroscopic investigations also show that the two unpaired electrons have orbital angular momentum about the bond axis. The magnitude of this orbital angular momentum is 

In Fig. 9.1 we indicate how the 2pz AOs on two equal second period element atoms may be combined to form one bonding and one antibonding a orbital, 2pob and 2p 7a respectively  

The two 2px orbitals may be also be combined to give a bonding and an antibonding MO  

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