Question

why in BeF2and BF3 are Be and B are electron defecient? can they fullfill their octet by making coordinate bond with F?

Answer 1

All atons need their outer shell filled with 8 electrons to get stability

B has 3 electrons in outermost shell and F has 7 electrons out of with 1 is unpaired.

The 1 unpaired electron find its pair with 1 electron of B each. As B have 3 electrons, 3 F find their pairs with each electron of B. So B now have 6 electrons in its vicinity(3 electron pairs) but it needs 4 pairs i.e. 8 electrons. So it is electron deficient.

So Boron always have affinity for 2 more electrons )preferably lone pair of some other atom.

@skb