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The structural and electronic properties of selected compositions of Sn x Ti 1−x O 2 solid solutions (x = 0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard's law in the Sn x Ti 1−x O 2 system. Our results also account for the fact that chemical decomposition in Sn x Ti 1−x O 2 system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn x Ti 1−x O 2 solid solutions. Sn substitution for Ti in TiO 2 increases the oxidation-reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron-hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn x Ti 1−x O 2 solid solutions.

The bonding in transition metal complexes is usually rationalized based on molecular orbital schemes. Topological approaches such as the atoms in molecules (AIM) theory or the electron localization function (ELF) analysis provides an alternative interpretation of the bonding relying on a local description. These topological theories give a very convenient framework to achieve the partition of the molecular space in regions with chemical meaning such as atoms, bonds and lone pairs. In this work we review the possibility offered by ELF to investigate the bond in di-tri-and tetranuclear metal complexes containing metal-metal bond. In the case of bimetallic complexes with different nominal bond orders of formula M 2 (formamidinate) 4 , the metal-metal interaction is associated to a large electron fluctuation between the two metallic cores and interpreted in terms of simple resonance arguments. Such fluctuation between metals can not be invoked for the trinuclear Fe 3 (CO) 12 or the incomplete cuboidal [Mo 3 S 4 (PH 3 ) 6 Cl 3 ] 4+ complexes. The metal-metal interaction in these clusters is mostly characterized by multicenter bonding as is the case for the tetranuclear heterodimetallic cubane-type complexes resulting from the insertion of Cu or Ni into the previous Mo 3 S 4 complex.

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