Preference for bridging versus terminal ligands in magnesium dimers

Magnesium dimers play important roles in inorganic and organometallic chemistry. This study evaluates the inherent bridging ability of a range of different ligands in magnesium dimers. In the first part, the Cambridge Structural Database is interrogated to establish the frequency of different types of ligands found in bridging versus terminal positions in two key structural motifs: one in which there are two bridging ligands (the D2h “Mg2(μ-X2)” structure); the other in which there are three bridging ligands (the C3v “Mg2(μ-X3)” structure). The most striking finding from the database search is the overwhelming preference for magnesium dimers possessing two bridging ligands. The most common bridging ligands are C-, N-, and O-based. In the second part, DFT calculations (at the B3LYP/6-311+G(d) level of theory) are carried out to examine a wider range of structural types for dimers consisting of the stoichiometries Mg2Cl3R and Mg2Cl2R2, where R = CH3, SiH3, NH2, PH2, OH, SH, CH2CH3, CH=CH2, C≡CH, Ph, OAc, F and Br. Consistent with the database search, the most stable magnesium dimers are those that contain two bridging ligands. Furthermore, it was demonstrated that the electronic effect of the bridging ligands is important in influencing the stability of the magnesium dimers. The preference for a bridging ligand, which reflects its ability to stabilize a magnesium dimer, follows the order: OH > NH2 > C≡CH > SH > Ph > Br > PH2 = CH=CH2 > CH2CH3 > CH3 > SiH3. Finally, the role that the ether solvent Me2O has on the stability of isomeric Mg2Cl2Me2 dimers was studied. It was found that the first solvent molecule stabilizes the dimers, while the second solvent molecule can either have a stabilizing or destabilizing effect, depending on the isomer structure.