Number of electron deficient molecules among the following
${\mathrm{PH}}_3,{ \mathrm{B}}_2{\mathrm{H}}_6,{\mathrm{CCl}}_4,{\mathrm{NH}}_3,\mathrm{NaF}$ and ${\mathrm{BCl}}_3$ is

In BCl₃, Boron has 3 valence electrons, after forming bonds with chlorine electrons around it increases to 6, but it still is short of 2 electrons. In B₂H₆, B forms 3C -2e^– bond and has less number  of electrons. Hence, these are electron deficient species.