What is the difference between the structure of AlCl3 and diborane?

The hydrogen bridged dimer B2H6 contains two three centre (3C-2e bond), two electron bonds, A three-centre bond uses two electron to link three atoms, and  four two centre, two electron bond (2C-2e). 3C-2e bridge bond is perpendicular to plane in which 2C-2e bond present.

In contrast the bridge bonding in Al₂Cl₆ contains two three centre (3C-4e bond), four electron bonds  which can be described in terms of electron pair bond in which a chlorine atom bonded to one aluminium atom act as a Lewis base by donating a lone pair of electrons to the aluminium atom which acts as a Lewis acid.