Theoretical Study On The Aromaticity Of Several Transition-metal Clusters

The aromaticity of organic compounds has been studied extensively. As the widely spread of aromaticity, the study of the aromaticity to the metal clusters of inorganic compounds increased, but there were few application to the transition-metal clusters. The stable isomers of the transition-metal clusters: Nb_n(n≤4), Co_n(n≤4), Nb_xCo_y(x+y≤8), Mn_x (3≤x≤6), V_xCo_x (x=2-4) were investigated by Density Functional Theory. Their aromaticity, energy and bonding properties were calculated. It was hoped to be better understand the special properties of these transition-metal clusters. The following conclusions have been drawn from our calculations:(1) Compared to the organic and inorganic clusters, the bonding properties were more complex for the transition-metal clusters, which were attributed to d orbitals. It was found that the aromaticity of the transition-metal clusters is stronger than that of the organic and inorganic clusters. The aromaticity of the organic and inorganic clusters was attributed to the contribution of s and p orbitals, but there were contributions from d orbitals for the transition-metal clusters. So it had three type of aromaticity:σ,Ï€andδfor the transition-metal clusters.(2)For a certain transition-metal cluster, it was found that the stable isomers with low energy had more aromaticity than that of unstable isomers. This was helpful for our study of the stability of the clusters. This phenomena is observed in transition-metal clusters Nb_n (n≤4), Co_n(n≤4), Nb_xCo_y (x + y≤8) and V_xCo_x (x = 2-4). The Nb₄Co₄ (T_d) and V₄CO₄ (T_d) clusters of high symmetry structure is close shell, they have the same characteristic of high aromaticity and stable structure. Some isomers of the Mn_x (3≤x≤6) clusters have not the phenomena of low energy and high aromaticity, because the Mn_x clusters have many d electrons, which are leading to high spin states and complex electron structure. At the same time, we found there have differend aromaticity with the same Mn_x(3≤x≤6) clusters, they have same structure but they have differend multi-states. The aromaticity of the cluster is the biggest that the multi-states of the clusters equal to one. With the multiplicity increase in the clusters, the electron spin states increase too, it lead to the structure of the clusters is not steady, then the aromaticity of the clusters decline. But with the multiplicity increase, there are not obvious rule with the magnetism and the aromaticity of the clusters.