Double aromaticity in transition metal centered double-ring boron clusters M@B2n (M = Ti, Cr, Fe, Ni, Zn; n=6, 7, 8)

It is well known that double-ring boron clusters have got the special double aromaticity with delocalized pi orbitals in two directions (tangential and radial), which are potential ligands centered by a transition metal. In this article, the transition metal centered double-ring boron clusters M@ B-2n (M = Ti, Cr, Fe, Ni, Zn; n = 6, 7, 8) are theoretically investigated by density functional theory calculations. These endohedral compounds have also got double aromaticity in both tangential and radial directions. Interestingly, the tangential delocalized pi orbitals of boron ligands following the Huckle's (4n + 2) rule do not interact with the central metal, while the radial pi orbitals of boron ligands are bonded with the central mental to form spd-pi endohedral bonding. The spd-pi endohedral bonding follows the 18e-principle in Ni@ B-14 and Fe@ B-16. However, due to the flat shape of the compounds, 14e (Cr@ B-14) and 16e (Ni@ B-12) can also be electronically very stable where the energy levels of the spd-p orbitals delocalized in z-direction rise up. This intriguing bonding model makes sense in further study of the boron chemistry. (C) 2014 AIP Publishing LLC.