Process for using finely divided highly reactive magnesium

In the formation of a compound including a --C--Mg--X moiety wherein X denotes a heteroatom selected from the group consisting of halogen, oxygen, sulfur, nitrogen, phosphorus, by reacting magnesium with a compound containing a poorly reacting --C--X moiety, the improvement which comprises using magnesium produced by thermally decomposing a magnesium-containing compound selected from the group consisting of

• (i) a magnesium anthracene and/or its derivatives prepared from magnesium and anthracene and/or its alkyl or phenyl derivatives,
• (ii) a magnesium butadiene and/or its alkyl or phenyl derivatives prepared from magnesium and a conjugated diene having the general formula R.sup.1 --CH.dbd.CR.sup.2 --CH.dbd.CH--R.sup.3, wherein R.sup.1, R.sup.2 and R.sup.3 are the same or different and represent H, a linear or branched alkyl group having from 1 to 6 carbon atoms or a phenyl residue, and
• (iii) a magnesium hydride-prepared from magnesium and hydrogen either in the presence of homogenous catalysts comprising a halide of a metal of the Subgroups IV to VII of the Periodic System and an organomagnesium compound or a magnesium hydride and in the presence of a polycyclic aromatic or a tertiary amine, after hydrogenating magnesium with magnesium anthracene or magnesium diene, said thermal decomposition taking place at a pressure from 10.sup.-6 to 1 bar and in the absence or presence of an organic aprotic solvent, either the decomposition being carried out in the presence of a co-reactant being added only after completion of the precipitation of the highly reactive magnesium, or in the absence of such co-reactant, the magnesium obtained by said decomposition being isolated as a highly reactive powder.