Rm Mg R'n where R and R' are the same or different C2 -C12 alkyl groups, preferably C4 -C10 alkyl groups, more preferably C4 -C8 alkyl groups, and most preferably both R and R' are mostly butyl groups, and m and n are each 0, 1 or 2, providing that m+n is equal to the valence of Mg.