Interplay of Hückel and Möbius Aromaticity in Metal-Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis*

CB6Al0/+: Planar hexacoordinate boron (phB) in the global minimum structure

Herein, we report for the first time the presence of a planar hexacoordinate boron (phB) atom in the global minimum energy structure of a neutral cluster system. The potential energy surface (PES) has been explored for CB₆Al^0/+/- systems using density functional theory (DFT). The global minima of CB₆Al (1a) and CB₆Al⁺ (1b) contain a phB center. However, the global minimum of CB₆Al^- (1c) does not have a phB atom. The CCSD(T)/aug-cc-pVTZ level of theory has been applied to compute the relative energies of the low-lying isomers with respect to the 1a and 1b structures of CB₆Al and CB₆Al⁺ systems, respectively. The exploration of the PES of CB₆^0/+/- systems indicates that the global minima do not contain a phB atom. However, the incorporation of an aluminium (Al) atom into the CB₆ moiety produces structures containing a phB center in the CB₆Al^0/+ systems. Hence, the Al metal has an important role in attaining a planar geometry having a hexacoordinate boron center. The dynamical stability of CB₆Al (1a) and CB₆Al⁺ (1b) was confirmed from the atom-centered density matrix propagation (ADMP) simulation over 20 ps of time at temperatures of 300 K and 400 K. The natural charge computations showed that the charges on the phB are almost zero in both systems. The 1a structure has σ/π-dual aromaticity as predicted from the nucleus independent chemical shift (NICS) values and the gauge-including magnetically induced ring current (GIMIC).