Understanding the influence of terminal ligands on the electronic structure and bonding nature in [Re6(μ3-Q8)](2+) clusters.

Since the synthesis of the first molecular cluster [Re6(μ3-Q8)X6](4-), the substitutional lability of the terminal ligands prompted new developments in their chemistry, making these molecular clusters a reasonable point of departure for building new materials. The development of novel inorganic materials of technological interest certainly requires an understanding of the electronic structure, bonding, spectroscopy, photophysical and structural properties of these clusters. Taking into account the potential applications in material sciences and the lack of systematization in the study of these kinds of clusters, the proposal of the present work is to perform a detailed theoretical study of the [Re6(μ3-Q8)X6](4-) (Q = S(2-), Se(2-), Te(2-); X = F(-), Cl(-), Br(-), I(-), CN(-), NC(-), SCN(-), NCS(-), OCN(-), NCO(-)) clusters based on the detailed description of the electronic structure of these complexes and the bonding nature between the [Re6(μ3-Q8)](2+) core and several donor-acceptor peripheral ligands. All this work was developed on the framework of the relativistic density functional theory, in which relativistic effects were incorporated by means of a two-component Hamiltonian with the zeroth-order regular approximation. To describe the relative stability of these complexes, we employed the global descriptors of chemical hardness and softness introduced by Pearson. Moreover, an analysis of bonding energetics was performed by combining a fragment approach to the molecular structure with the decomposition of the total bonding energy according to the Morokuma-Ziegler energy partitioning scheme. After an analysis of these results, we found in all cases an extensive ionic character in the bonding between the core and each peripheral ligand. The interaction between the halide ligand and the core gives about 75% ionic character, whereas the other ligands show a more covalent interaction due to effective synergic mechanisms. We conclude that the most stable clusters are those that present the stronger σ-donor terminal ligands, whereas the cluster stability starts to decrease when the π-acceptor effect will be stronger; this fact is directly related to the terminal ligand lability and the strong electrophilic character of the [Re6(μ3-Q8)](2+) core.