Organomagnesium clusters: Structure, stability, and bonding in archetypal models

We have studied the molecular structure and the nature of the chemical bond in the monomers and tetramers of the Grignard reagent CH3MgCl as well as MgX2 (X = H, Cl, and CH3) at the BP86/TZ2P level of theory. For the tetramers, we discuss the stability of three possible molecular structures of C2h, D2h, and Td symmetry. The most stable structure for (MgCl2)4 is D2h, the one for (MgH2)4 is C2h, and that of (CH3MgCl)4 is Td. 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 ΔVelstat (60% of all bonding terms ΔVelstat + ∆Eoi) although the orbital interaction ∆Eoi also provides an important contribution (40%).