The hydrogen bridged dimer $B_2H_6$ contains two three centre, two electron bonds, A three-centre bond uses two electron to link three atoms, In contrast the bonding in the halogen bridged $AlCl_3$ 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.
