What are electron deficient compounds? Are BCl3 and SiCl4 electron deficient species?

In an electron-deficient compound, the octet of electrons is not complete, i.e., the central metal atom has an incomplete octet. Therefore, it needs electrons to complete its octet.

(i) $BC{l}_3$

$BC{l}_3$ is an appropriate example of an electron-deficient compound. B has 3 valence electrons. After forming three covalent bonds with chlorine, the number of electrons around it increases to 6. However, it is still short of two electrons to complete its octet.

(ii) $SiC{l}_4$

The electronic configuration of silicon is $n{s}^2$ $n{p}^2$ . This indicates that it has four valence electrons. After it forms four covalent bonds with four chlorine atoms, its electron count increases to eight. Thus, $SiC{l}_4$ is not an electron-deficient compound.