The Trigonal Bipyramidal MN 3 M Species: A New Kind of Aromatic Complex Containing a Multiple‐Fold Aromatic N 3 Subunit

A new kind of aromatic trigonal bipyramidal MN 3 M (M=Be, B, Mg, Al, and Ca) species, with all real frequencies, is obtained at the MP2/6‐311+G(3d) level. The nucleus‐independent chemical shift values are −102.16 ppm for the N 3 3− ring, and −74.09, −79.39, −65.06, −74.44, and −62.33 ppm (at the geometrical center of the trigonal bipyramid) for BeN 3 Be, BN 3 B, MgN 3 Mg, AlN 3 Al, and CaN 3 Ca, respectively. Molecular orbital analysis indicates that the regular triangular N 3 3− ring and each MN 3 M species have three aromatic six‐electron systems (π, σ p , and σ s ) and exhibit threefold aromaticity. The CaN 3 Ca species has a very low vertical ionization energy of 3.64 eV at the CCSD(T)/6‐311+G(3d) level, which is even lower than the ionization energy (3.9 eV) of the Cs atom. Therefore, CaN 3 Ca can be considered as a new superalkali species. A further study on the CaN 3 CaCl molecule confirms the superalkali characteristics of CaN 3 Ca. Two interesting phenomena are explored in the MN 3 M species: the delocalized electron cloud of the N 3 subunit is elongated by two M cations, and the electron clouds of two M cations are distended by the N 3 3− ring.