boron and hydrogen have almost equal values of electronegativity
pπ−dπ back bonding is possible in B2H6 but not in B2Cl6
boron and chloride have almost equal atomic sizes
small hydrogen atoms can easily fit in between boron atoms but large chlorine atoms do not.
Hydride of boron occurs as B2H6 but B2Cl6 does not exist. This is because small hydrogen atoms can easily fit in between boron atoms but large chlorine atoms do not.