Theoretical Studies On The Structure And Chemical Bonding Of Group VB Transition Metal Boron Clusters

Borides have many industrial applications in refractory materials, superhard materials, nano-materials and electrode materials due to the specific physicochemical properties of boron. In order to further explore the potential applications of borides and then figure out the internal relations between the excellent physicochemical properties and the structural and electronic properties of borides, we report the theoretical studies on a series of group VB transition metal boron clusters. A summary of the work is given as following:1. Density functional theory (DFT) calculations are performed to investigate the structural and electronic properties of a series of di-tantalum boride clusters, Ta2Bx-/0 (x= 1-5). Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES) of the anionic low-lying isomers. A trend of structural evolution is observed with the number of boron increasing in the Ta2Bx-/0 (x= 1-5) clusters. Started from the isosceles triangle structure of Ta2B-, the global minimum of anionic Ta2B2- is characterized by a distorted tetrahedron with a Ta-Ta bond cross interplaying upon to the B-B moiety. Based on the structure of anionic Ta2B2-, the following additional boron atoms are added to extend the boron chain on the equatorial plane. The global minima of the neutral clusters follow the same pattern to the respective anions.2. The structural and electronic properties of a series of di-niobium boride clusters, Nb2Bx-/0 (x=1-5), are investigated using density functional theory (DFT) calculations. Showing a great accordance with the Ta2Bx-(x=1-5) clusters, the structural evolution of the Nb2Bx-/0 (x=1-5) series starts from the isosceles triangle structure, and shows a trend to extend the boron chain on the equatorial plane with the function of boron content. The differences occur in the potential energy surfaces and important vertical detachment energies between Mb2Bx-/0 (x= 1-5) and Ta2Bx-/0 (x= 1-5) series, may be caused by different metal-boron bonding properties and other more complicated factors.3. Based on the studies of Nb2Bx-/0 (x=1-5) and Ta2Bx-/0 (x= 1-5) clusters, a theoretical study is carried out on the structural and electronic properties of M2B*-/0 (M= V, Nb, Ta; x= 6-7) clusters. The lowest energy structures of M2B-/0(M= V, Nb, Ta; x= 6-7) clusters are calculated to be bi-pyramid structures with high symmetry, which are characterized by enclosed boron rings on the equatorial plane. Molecular orbital analyses show that the enclosed boron rings in the bi-pyramid structures are essential factors to stabilize the structures. Therefore, the unique boron rings should be important bridges between the study of gas-phase cluster and bulk boron, which can provide insights into the design of novel boron-based metal complexes.