Electron deficient species are those in which the central atom in their molecules as the tendency to accept on or more electron pairs. They are also known as Lewis acids .BCl3 and SiCl4 both are electron deficient species. Since, in BCl3 and B atom as only 6 electrons therefore, it is an electron deficient compound. In SiCl4 the central atom as 8 electrons but it can expand it covalence beyond 4 due to the presence of d-orbital. Thus .SiCl4 should also be considered as electron deficient spaces.
