BCl₃ exist but BH₃ does not. Give reason

BH₃ is an electron deficient compound which has an empty p orbital or we can say 6 electron in its outermost shell. As the size of Hydrogen is small therefore it facilitates the dimerisation of BH₃ through 3 entered 2 electron bond. This dimerisation results in the formation of B₂H₆ ( diborane ), where the single hydrogen is shared between two boron atoms.
On the other hand, BCl₃ does not dimeries due to its small size. As the size of boron is small therefore it is unable to accommodate 4 large chlorine atoms at the same time.