I believe this is a conceptual question and might sound silly, but please help me with it:

I can understand when ions like $\text{Co}^{3+},\text{Mg}^{2+}$ are called electron deficient, because they have no valence electrons, but how do I call a compound like $\text{AlCl}_3$ electron deficient? What's the basis for calling such a compound electron deficient?

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    $\begingroup$ Is aluminium's octet complete? $\endgroup$ – Avnish Kabaj Feb 4 '19 at 10:44
  • $\begingroup$ Yeah, I believe aluminium's octet is complete. $\endgroup$ – Dhrubajyoti Ghosh Feb 4 '19 at 11:05
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    $\begingroup$ @DhrubajyotiGhosh No, it's not. Count the valence electrons again. $\endgroup$ – andselisk Feb 4 '19 at 12:07
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    $\begingroup$ Being electron-deficient would imply that the molecule possesses vacant MO which can feasibly accept electrons from an effective Lewis base. And yes, AlCl3 has an incomplete octect(6 electons). $\endgroup$ – William R. Ebenezer Feb 4 '19 at 12:13
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    $\begingroup$ @DhrubajyotiGhosh No, it's not silly, it's fine. Only the one who does nothing is never mistaken. You can post an answer to your question if you solved the issue yourself:) $\endgroup$ – andselisk Feb 5 '19 at 13:05