Twenty years ago, the molecular materials community accomplished a major breakthrough with the report of the first single component molecular metal (SCMM). Previously it was believed that molecular metals had to be composed of a pair of partially oxidized species, a condition necessary to guarantee the formation of the conduction band and the generation of charge carriers. However, by the turn of the millennium a new paradigm arose with the discovery of metallic behaviour in two neutral complexes, [Ni(tmdt)₂] (tmdt: trimethylenetetrathiafulvalenedithiolate) and [Au(α-tpdt)₂] (α-tpdt: 2,3-thiophenedithiolate). Since then, this topic gathered the attention of the scientific community and the number of available SCMM in literature consequentially increased. By their nature, neutral bis(1,2-dithiolene) and bis(1,2-diselenolene) transition metal (BTM) complexes still remain, by far, the best candidates to obtain a SCMM. This review will focus on these materials, their design, synthesis, and properties. Based on their molecular-crystal-band structures, an analysis of the requirements to prepare a SCMM, will also be address.

The Au and Ni monoanionic complexes of tert-butyl and diisopropyl substituted thiophenedithiolate ligands, [M(α-tb-tpdt)₂] and [M(α-dp-tpdt)₂], were synthesized and characterized namely by single crystal X-ray diffraction and magnetic susceptibility measurements. These complexes, prepared in a first step as monoanionic species, are easier to oxidize than the related non-substituted thiophenedithiolates and could be obtained also as stable neutral species. As expected, the peripheral alkyl groups in the ligands also confer a high solubility to the complexes in common organic solvents. The neutral gold complex [Au(α-tb-tpdt)₂] presents a significant ligand asymmetry indicative of unpaired electron localization in one ligand at variance with [Au(α-dp-tpdt)₂] that is within experimental uncertainty fully symmetric illustrating the role of intermolecular interactions in the stabilization of SOMO⋯SOMO interactions. While in [Au(α-tb-tpdt)₂] a significant intermolecular interaction between paramagnetic molecules is possible leading to diamagnetic dimers of molecules, in [Au(α-dp-tpdt)₂] the bulkier substituents prevent the intermolecular interactions, leading to a regular stacking of molecules in symmetrical configuration. The regular stacks of paramagnetic [Au(α-dp-tpdt)₂] units behave, at high temperatures, as antiferromagnetic chains undergoing an AFM transition at ca. 25 K.

Spin-ladders are fascinating quantum magnetic systems. Their structural and magnetic properties and the possibilities offered by molecular units of different nature and crystal engineering tools to prepare molecular spin-ladders are discussed.

The cytotoxic activity of four sets of camphorimine complexes based on the Cu(I), Cu(II), Ag(I), and Au(I) metal sites were assessed against the cisplatin-sensitive A2780 and OVCAR3 ovarian cancer cells. The results showed that the gold complexes were ca. one order of magnitude more active than the silver complexes, which in turn were ca. one order of magnitude more active than the copper complexes. An important finding was that the cytotoxic activity of the Ag(I) and Au(I) camphorimine complexes was higher than that of cisplatin. Another relevant aspect was that the camphorimine complexes did not interact significantly with DNA, in contrast with cisplatin. The cytotoxic activity of the camphorimine complexes displayed a direct relationship with the cellular uptake by OVCAR3 cells, as ascertained by PIXE (particle-induced X-ray emission). The levels of ROS (reactive oxygen species) formation exhibited an inverse relationship with the reduction potentials for the complexes with the same metal, as assessed by cyclic voltammetry. In order to gain insight into the toxicity of the complexes, their cytotoxicity toward nontumoral cells (HDF and V79 fibroblasts) was evaluated. The in vivo cytotoxicity of complex 5 using the nematode Caenorhabditis elegans was also assessed. The silver camphorimine complexes displayed the highest selectivity coefficients (activity vs. toxicity).