We have studied the molecular structure and the nature of the chemical bond in the monomers and tetramers of the Grignard reagent CH(3)MgCl as well as MgX(2) (X = H, Cl, and CH(3)) at the BP86/TZ2P level of theory. For the tetramers, we discuss the stability of three possible molecular structures of C(2h), D(2h), and T(d) symmetry. The most stable structure for (MgCl(2))(4) is D(2h), the one for (MgH(2))(4) is C(2h), and that of (CH(3)MgCl)(4) is T(d). The latter is 38 kcal/mol more stable with chlorines in bridge positions and methyl groups coordinated to a Mg vertex than vice versa. We find through a quantitative energy decomposition analysis (EDA) that the tetramerization energy is predominantly composed of electrostatic attraction Delta V(elstat) (60% of all bonding terms Delta V(elstat) + Delta E(oi)) although the orbital interaction Delta E(oi) also provides an important contribution (40%).