IR spectra of products of the reactions of methyl halogen molecules CH3Cl and CH3Br with Mg have been studied under matrix isolation conditions. Comparison of the spectra with ab initio frequencies and intensities allows us to identify the primary reaction products as monomagnesium Grignard complexes CH3MgX = (X Cl, Br) and therefore reject the possibility of creation of energetically more stable bimagnesium species CH3-MgMgX. Those species according to ab initio calculations should be bound with respect to the decomposition CH3MgX + Mg. Ab initio calculations have been carried out at several levels, including the large basis set MP2 and DFT treatments. The scaling factors, which bring the computed harmonic frequencies into correspondence with the experimental band positions, have been deduced. It is shown that, unlike the majority of organic molecules, organometallic complexes should be considered more carefully, in the sense that the scaling factors of the advanced MP2 and DFT procedures are not as close to unity as is believed.