Disclosed is a Ziegler-Natta catalyst for olefin polymerization comprising a metal cation of Group 4 or 14 of the Periodic Table (IUPAC 1976) that is coordinated to a. a carborane dianion group; b. a heteroatom anion, said heteroatom being preferably in the terminal position of a hydrocarbyl bridge comprising at least two carbon atoms, wherein at least one bridging carbon atom may be replaced by Si; and c. a valence group.