Intramolecular magnesium bonds in malonaldehyde-like systems: a critical view of the resonance-assisted phenomena

Through the use of high-level G4-theory calculations, we have investigated the structure, stability, and bonding of a set of Mg derivatives formed by replacing the –OH group of malonaldehyde or only the hydrogen atom of this group by a –MgH group. To give insight into the resonance-assisted phenomenon, which might be involved in the stabilization of these compounds, we also included the corresponding saturated analogs in our survey. The effect of the rigidity of the molecular framework was considered by analyzing the Mg derivatives of (Z)-4-(hydroxymethylene)cyclobut-2-enone, obtained through the same substitutions mentioned above. The effect of replacing the carbonyl group by an imino group was also contemplated. In all cases, the global minimum is a cyclic conformer stabilized through the formation of rather strong intramolecular magnesium bonds. The strength of these interactions is directly related with the intrinsic basicity of the carbonyl group (or the imino group) and the intrinsic acidity of the –MgH group, rather than with a resonance-assisted phenomenon. As a matter of fact, for all the investigated systems, the conclusion is that resonance in the cyclic conformer is directly correlated with the strength of the intramolecular magnesium bond, and not vice versa. Interestingly, the strength and characteristics of these interactions for these Mg-containing derivatives are very similar to those of the corresponding Be-containing analogs.