Cluster Grignard reagents C6H5Mg4F have been obtained by direct magnesium cluster insertion into C-F bond in fluorobenzene with magnesium vapors co-condensates at low temperature. The appearance of metal cluster in organomagnesium molecule leads to unusual non-transition metal cluster compounds catalytic properties studied on halogen exchange reaction RF + R'Cl --> RCl + R'F as example. Cluster Grignard reagents catalyze the fluorobenzene reaction with alkyl chlorides due to reversible magnesium cluster insertion into C-F and C-Cl bonds. (C) 2002 Elsevier Science B.V All rights reserved.