Number of electron deficient molecules among the following PH _ 3 ,...

A molecule is said to be electron deficient because there are not enough valence electrons to form the expected number of covalent bonds. For example, in ${B}_{2}{H}_{6}$, there are only 12 electrons, three from each boron atoms and six from hydrogen, while ethane possesses 14 such electrons.

Also in ${BCl}_{3}$, B has six electrons in its outermost orbit. So, the number of electron deficient molecules are 2.