Compounds which have very few valence electrons (less than 8) or 8 valence electrons with empty‘d’ orbitals for forming covalent bonds between the atoms are called electron deficient compounds.
Compounds having highly polarized bonds due to highly electronegative ligands and whose valence shell are not completely occupied and can act as Lewis acids can also be called electron deficient compounds.
‘Yes’ BCl3 (boron trifluoride) is electron deficient compound but SiCl4 (silicon tetrafluoride) is not an electron deficient compound.
BCl3 – In boron trifluoride, boron has 3 valence electrons. After forming covalent bonds with three chlorine atoms boron will have 6 valence electrons around it. Even then boron is short of 2 electrons to form the octet configuration.
SiCl4 – In silicon tetrafluoride, silicon has 4 valence electrons. After forming covalent bonds with four chlorine atoms silicon will have 8 valence electrons around it.
Silicon cannot extend its covalency beyond 4, as it does not have‘d’ orbitals. Hence SiCl4 is not an electron deficient compound.