Stabilization of Boron and Carbon Clusters with Transition Metal Coordination - An Electron Density and DFT Study

Using density functional theory (DFT) calculations, we report here recent developments on the encapsulation of a transition metal (M) atom in boron clusters in the size range of up to 24 boron atoms and carbon monocyclic ring structures in the size range of 9–12 carbon atoms. In particular, we discuss our findings of M-atom encapsulated rings/wheels (8 and 9 boron atoms), drums (14, 16, and 18 boron atoms), and fullerene-like cage structures with 20–24 boron atoms in contrast to the fullerene-like structure of pure boron with about 40 atoms. Dimer-capped drum-shaped structures are favored for neutral Cr@B 18 , Mo@B 20 , and W@B 20 clusters, whereas a drum-shaped structure is preferred for M-atom encapsulated B 14 , B 16 , as well as neutral, cation, and anion of Mo@B 18 and W@B 18 . Further, B 20 is the smallest cage for Cr encapsulation, while B 22 is the smallest symmetric cage for Mo and W encapsulation. Symmetric cage structures are also obtained for Mo@B 24 and W@B 24 . A detailed analysis of the bonding character and molecular orbitals (MO) suggests that Cr@B 18 , Cr@B 20 , M@B 22 (M = Cr, Mo, and W), and M@B 24 (M = Mo and W) cages are stabilized with 18 π-bonded valence electrons whereas the drum-shaped M@B 18 (M = Mo and W) clusters are stabilized by 20 π-bonded valence electrons. Calculations with PBE0 functional in Gaussian 09 code show that in all cases of neutral clusters, there is a large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap. Further results of the encapsulation of an M atom in a small carbon cluster to form wheel structures similar to those of boron are discussed.