Electron deficient compounds are those compounds in which there are only six electrons in the valency shell of central atom after the formation of the molecule. Electron deficient bonds are often described as 3-center-2-electron bonds.
BCl3 is electron deficient compound because boron has only 6 valence electrons and is short of 2 electrons. Therefore, it can accept a pair of electrons from nucleophiles. SiCl4 is not electron deficient compound because silicon has 8 valence electrons.
