back bondin is ususlly seen in compunds havin elemts wich hav empty d orbital

for xample in PF3

P has an empty d orbital

F elctrons r congested in d small structure of F due 2 wich thir is repulsion n these elctrons try 2 move out

 

thus as P has an empty d orbital these elctrons can accomodate that place easily

 

in PF3 usually d bond is betwn P n F that 2 2wrds F coz itz electronegative ( more then P) bt here thiers also oen more bond wich goes frm F to P coz of these active elctrons

 

this is knw as back bondin

itz said ' back' coz itz in reverse direction

itzzz also called Ppi-Dpi back bondin wen d orbitals takin part r P n D

 

hope u get it!!