The pre-formed clathrate is then reacted, preferably under stirring, with the organotransi- tion metal compound of formula (I), as described above, under inert atmosphere of for example N 2 or Argon at temperatures between 10 and 40 0 C, preferably between 15 and 35??C and more preferably between 20 and 30 0 C, for example at room temperature.