Transitionmetalcomplexes obtained by reduction of transitionmetalchlorides with magnesium catalyze a slow insertion reaction of magnesium into carbon-oxygen bonds of tetrahydrofuran and other ethers. The order of reactivity of the transitionmetals tested is Ti ≈ Co > V > Cr > Zr > Rh > Mn > Nb > Fe ≈ Ni. The effects on the reaction of modifications of the catalysts are discussed.