The invention relates to a complex product containing magnesium, halogen and alkoxy, which has the following composition: MgpXq(OR)2p-q wherein X is a halogen, R is an alkyl group having from 1 to 20 carbon atoms, p is from 2 to 20 and q is from 1 to (2p−1). More specifically the complex product is a complex of the formula MgCl2.[Mg(OR)2]2 wherein R is an alkyl having from 4 to 10 carbon atoms. The complex is soluble in non-polar solvents and can be used in the preparation of transition metal components of Ziegler-Natta catalyst systems.