A nice question
both silicon and carbon belong to group 14 and have 4 electrons in the outermost shell. Their general valency is 4 , but silicon and other elements show higher oxidation states also.
Only carbon is the element in group 14 elements that do not have vacant d-orbital.
All other elements like silicon ,germanium ,tin And lead Have empty d-orbitals in which they can accommodate electrons from Other electron rich species and can extend their oxidation states or Valencies for making the complex molecules.
Here in the above diagram we can see the two floride ions can share electrons in vacant 3d orbital . Carbon do not have any vacent d-orbital for accomodating the extra electrons that’s why it is unable to make such compund.
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