Square-planar /8 metal dithiolenes

Chapter 2- Transition metal dithiolenes are versatile complexes eapable of a wide range of oxidation states, coordination geometries, and magnetie moments. As a eonsequence, these complexes have been widely studied as building bloeks for erystalline molecttlar materials. Particularly successful are the square-planar metal dithiolenes (Chart 1), from which materials have been produeed that exhibit condueting, magnetic, and nonlinear optieal properties, as well as supereonduetivity in some cases. " In their application to molecular- based... 

FIGURE 1. Frontier MOs in square-planar metal-dithiolene complexes formed by the in-phase (i.p.) and out-of phase (o.o.p.) combinations of C2S2 orbitals perturbed by the metal. The d metal orbitals stabilized relative to ligand orbitals lead to an inverted bonding scheme . Reprinted with permission from Reference 12. Copyright 2004 American Chemical Society... 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

By far, the most complete and rigorous vibrational assignments that are available for metallodithiolenes are for square-planar bis(dithiolene) metal... 

Square-planar bis(dithiolene) complexes have also been the subject of theoretical investigations (43-46). For example, density functional theory (DFT) calculations indicated that the highest occupied molecular orbital (HOMO) for Ni(S2C2H2)2 is primarily a ligand-based orbital comprising of four 3pz orbitals of sulfur, perpendicular to the molecular xy plane, and four 2p-orbitals of carbon with opposite phases. The lowest unoccupied molecular orbital (LUMO) is a mix of ligand-metal orbitals, but still mostly of the ligand... 

Although the experimental evidence suggests that the observed photogeneration of hydrogen may not involve homogeneous metal dithiolene photocatalysts, a theoretical study by Alvarez and Hoffmann addressed possible mechanisms for hydrogen elimination from ds square-planar bis(dithiolene) complexes (77). Concerted elimination of H2 from protonated sulfur atoms in the complex was proposed to be a thermally forbidden but photochemically allowed pathway, and protonation of a metal hydrido complex was also considered, as shown in Scheme 3. 

The combination of dithiolene and diimine chelating ligands in square-planar ds complexes gives rise to a unique CT excited state, and complexes of this class have been the subject of a rich and growing amount of research in recent years. The Pt(diimine)(dithiolene) complexes were among the earliest examples of emission from square-planar metal complexes in fluid solution. Luminescence from room temperature solutions of Pt(diimine)(mnt) complexes with diimine = bpy, phen, or an alkyl- or aryl-substituted derivative was reported in 1990 by Zuleta et al. (98, 99), following an initial report on similar complexes with a 1,1-dithiolate. 

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand square-planar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL CT excited state (14, 129). 

Homoleptic bis(dithiolene) complexes of d10 metal ions such as Cu(I), Zn(II), Cd(II), and Hg(II) are known. The tetrahedral zinc bis(dithiolene) complexes are among the best studied, and display much of the same CT photochemistry as the square-planar bis(dithiolene) complexes of the d8 metal ions (65, 66). The use of... 

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