Small Magnesium Clusters: Between van der Waals and Valence Bonds

The magnesium molecules Mg2, Mg3, Mg4, and Mg5 have been isolated in rare-gas matrixes at 8 K and characterized for the first time by Raman spectroscopy. The vibrational frequencies were compared with those obtained by density functional theory calculations. The fundamental frequency of the dimer was observed at 97 cm−1 (Ar matrix), and a bond length of d(Mg−Mg) = 340.8 pm was calculated. The Mg3 cluster possesses a triangular structure (D 3h ) with calculated d(Mg−Mg) = 317.8 pm. For tetramagnesium, a tetrahedral structure (T d ) with d(Mg−Mg) = 301.3 pm is predicted. The Mg5 molecule possesses a trigonal-bipyramidal structure (D 3h ) with d(Mg−Mg) = 293.1 and 322.5 pm, respectively. The red tetramagnesium cluster is an example of a matrix-isolated magic number cluster.