two centred three electron bond......three centered two electron bond
and case of AlCl3 is different AlCl3 uses pπ-dπ bonding also known as back bonding for its dimer structure
basically three centered two electron bond formed by hydrogen or methyl or phenyl groups as middle specie and stability order is
hydrogen > phenyl> methyl
but i dont think u will be needing this stuff
tell me if u want more details