The answer is C) BeCl2 because berliuim's valency is 4 and it needs 2 more electrons to complete its octect...
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can u give me an idea that what are electron deficient compound?
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Boron Chloride is the electron deficient compound. B Cl₃.  Boron halides BF₃, BCl₃ and B Br₃ all of them react with Lewis acids to form adducts. The reaction gives out a lot of heat. This tells that the compounds have affinity to electrons and so they react.

Electron deficient compounds have less valence electrons for the atoms in the structure. They have strongly   polarised (electro negative) bonds.  

The planar structure of Boron halide is readily converted in to the pyramidal structure. The nature of Pπ bonds give that property to Boron halides. These Boron halides have nature of Lewis acids. 

They form adducts with dimethyl sulfide:  BF3 . S(Me)2.  Also with Caesium Chloride:
       Cs Cl + B Cl3 => Cs B Cl4

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