A gas-phase study of the coordination of Mg2+ with oxygen- and nitrogen-containing ligands

An experimental study of the stability and coordination of oxygen- and nitrogen-containing ligands in association with Mg2+ in the gas phase has been undertaken. The ligands chosen exhibit a wide range of physical properties in terms of their ionization energies, dipole moments, and polarizabilities, and a simple electrostatic model reveals a semiquantitative trend between these properties and the ability of each ligand to stabilize Mg2+. The model clearly demonstrates why water is extremely effective at stabilizing Mg2+, and in this respect, CO2 also proves to be a good ligand. Evidence of a discrete first solvation shell is apparent only for those ligands which do not display hydrogen bonding. For water, methanol, and ethanol, hydrogen bonding leads to extended solvation units for which the boundaries are less obvious. However. for more complex alcohols, steric interactions appear to negate the influence of hydrogen bonding. Discrete solvation shells are observed for most aprotic ligands. and the optimum coordination number is 4. However, there is some slight variation in this value, mainly as a consequence of ligand size. Assuming Mg2+ to be a hard Lewis acid, the results are used to order the ligands in terms of how effective they are at stabilizing Mg2+ in their role as hard Lewis bases. Evidence of the first gas-phase Mg2+ bidentate metal complex is also provided.