Three-center bonding
Example of where the 2c bonding picture is not present is in diborane B2H6. The terminal B-H bonds can be defined in simple 2c terms, but the number of electrons available advised that each bridging hydrogen forms part of a 3c bond involving the two boron atoms. The MO method gives a simple interpretation. Four H atoms are disposed roughly tetrahedrally around each boron; this arrangement gives that sp3 hybrids are used. Two such hybrids form normal 2c bonds by overlap with the 1s AO on the terminal hydrogen's. The others are correlated as in 1 to form two 3c bridge bonds. In addition to B-H overlap there is some direct overlap between the boron hybrids, which gives some B-B bonding as well. The result is named as a three center two-electron (3c2e) bond. 3c2e bonds with bridging hydrogen occur in other circumstances, as like the normal form of BeH2, which has a polymeric chain structure with all H atoms in bridging positions.
Other groups such as methyl CH3 can do this, as in diametric aluminum methyl, Al2(CH3)6, which has a structure essentially same as B2H6 with CH3 in place of H.