Why ClF7 is not known but …

Although theoretically it is logical  to view a possibility of the existance  of ClF₇ simillar to  IF₇ , but practically   Cl F₇ is unknown and hence  , it does not exist .The following reasons support the non - existance of ClF₇ .

• Steric reasons -  Fitting seven electrons around a Cl atom is sterically impossible due to electron cloud -repulsion of Cl   over F atoms.
• Difference between atomic sizes -  Both the compounds ( ie. IF₇ and ClF₇ ) are interhalogen - compounds which are formed when    

                 (1)  electronegativity difference between the combining atoms is large

                 (2)  the difference in atomic sizes are great. 

            The difference in atomic sizes of  Cl and  F  is  is  lesser than that of  I  &  F .  In case of  F their happens to be a much                      greater difference in its atomic sizes when  compared to that of I .So, small F atoms can be easily accomodated around                    iodine atoms to form  IF₇ . Thus , IF₇  exists , while ClF₇  is not known.