Cyclic Boron Clusters Enclosing Planar Hypercoordinate Cobalt, Iron, and Nickel

Planar cyclic boron clusters with cobalt, iron, and nickel atoms at their centers-singlet D-8h CoB8-, D-9h FeB9-, CoB9, and NiB9+-are computed to be stable minima at the BP86/TZVPP DFT level. Stochastic searches of the singlet and triplet potential energy surfaces show the planar hypercoordinate D-8h CoB8- (1) and D-9h FeB9- (2) singlet isomers to be the global minima. Their double aromatic character with 6 pi and 10 radial electrons is documented by detailed NICSzz grid and CMO-NICSzz analyses at PW91/TZVPP. These results encourage gas phase investigations of these two exotic anions. Although isoelectronic with D-9h FeB9- (2), CoB9 and NiB9+ prefer nonplanar structures, triplet 3-aT for the former and singlet 4-a for the latter.