SiF 4 and SF 4 are not isostructural becuase SiF 4 is tetrahedral due to sp 3 -hybridisation of Si.

14 Si = 1s 2 , 2s 2 2p 6 , 3s 1 3p 3 (in excited state)

Hence, four equivalent sp 3 hybrid orbitals are obtained and they are oberlapped by four p-orbitals of four fluorine atoms on their axes. Thus it shows following structure :

SF 4 SiF 4

While SF 4 is not tetrahedral but it is distorted tetrahedral because in it S is sp 3 d hybrid.

16 s = 1s 2 , 2s 2 2p 6 , 3s 2 3p 2x 3p 1y 3p 1z

(In groud state)

= 1s 2 , 2s 2 2p 6 , 3s 2 3p 2x 3p 1y 3p 1z , 3d 1xy

(sp 3 d - hybridisation)

(In first excitation state)