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Aromaticity is a notion that appeared in the mid-nineteenth century to differentiate between unsaturated hydrocarbons and formally unsaturated benzene [1–3]. At the end of the nineteenth century it seemed that cyclicity was a necessary condition for differentiation between the two, but at the beginning of the twentieth century it turned out that the above assumption was not correct because cyclooctatetraene exhibited typical properties known for polyenes [4]. The essential property of b- zene-like compounds, often identified with aromatic compounds, was low react- ity. Hence thermodynamic stability was defined as resonance energy [5, 6] and was the first quantitative measure of aromaticity. Many theoretical approaches were proposed later to estimate this quantity, and now the criterion is often considered to be the most fundamental [7]. Almost at the same time, magnetic susceptibility was used to describe aromaticity [8, 9]. Consequently, many concepts based on mag- tism were developed, probably the most effective in assessment of aromaticity being nucleus independent chemical shift (NICS) [10] or Fowler’s maps of ring currents [11]. The criterion served Schleyer as a basis for a definition of aromat- ity: “Compounds which exhibit significantly exalted diamagnetic susceptibility are aromatic. Cyclic delocalisation may also result in bond length equalization, abn- mal chemical shifts and magnetic anisotropies, as well as chemical and physical properties which reflect energetic stabilisation”[12].

Heterocyclic compounds --- Aromaticity (Chemistry) --- Biochemistry --- Organic Chemistry --- Chemistry --- Physical Sciences & Mathematics --- Heterocyclic compounds. --- Organic cyclic compounds. --- Compounds, Organic cyclic --- Cycloids, Mixed (Chemistry) --- Heteroatomic compounds --- Heterocycles --- Mixed cycloids (Chemistry) --- Chemistry. --- Organic chemistry. --- Organic Chemistry. --- Cyclic compounds --- Organic compounds --- Organic cyclic compounds --- Chemistry, Organic. --- Organic chemistry

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The chemistry of silicon has always been a field of major concern due to its proximity to carbon on the periodic table. From the molecular chemist's viewpoint, one of the most interesting differences between carbon and silicon is their divergent coordination behavior. In fact, silicon is prone to form hyper-coordinate organosilicon complexes, and, as conveyed by reports in the literature, highly sophisticated ligand systems are required to furnish low-coordinate organosilicon complexes. Tremendous progress in experimental, as well as computational, techniques has granted synthetic access to a broad range of coordination numbers for silicon, and the scientific endeavor, which was ongoing for decades, was rewarded with landmark discoveries in the field of organosilicon chemistry. Molecular congeners of silicon(0), as well as silicon oxides, were unveiled, and the prominent group 14 metalloid proved its applicability in homogenous catalysis as a supportive ligand or even as a center of catalytic activity. This book focuses on the most recent advances in the coordination chemistry of silicon with transition metals as well as main group elements, including the stabilization of low-valent silicon species through the coordination of electron donor ligands. Therefore, this book is associated with the development of novel synthetic methodologies, structural elucidations, bonding analysis, and also possible applications in catalysis or chemical transformations using related organosilicon compounds.

cluster --- molecular orbital analysis --- bond activation --- X-ray diffraction --- silsesquioxanes --- digermacyclobutadiene --- intermetallic bond --- germanium --- computational chemistry --- ?-electron systems --- isocyanide --- X-ray crystallography --- cyclic organopolysilane --- disilene --- ruthenium --- platinum --- DFT --- Photostability --- silicon surfaces --- stereochemistry --- palladium --- distorted coordination --- ²⁹Si NMR spectroscopy --- organosilicon --- disilanylene polymer --- Si–Cl activation --- adsorption --- AIM --- siliconoid --- nanoparticle --- disiloxane tetrols --- germylene --- hydrogen bonding --- TiO₂ --- dehydrogenative alkoxylation --- siloxanes --- 2-silylpyrrolidines --- bonding analysis --- ?-chloro-?-hydrooligosilane --- hydrido complex --- oxidative addition --- photoreaction --- template --- surface modification --- titanium --- bromosilylenes --- host-guest chemistry --- hydrogen bonds --- salt-free --- N-heterocyclic carbines --- silicon cluster --- condensation --- silyliumylidenes --- Baird’s rule --- N-heterocyclic carbenes --- reductant --- main group coordination chemistry --- molecular cage --- subvalent compounds --- isomerization --- silanetriols --- germathioacid chloride --- dehydrobromination --- N-heterocyclic carbene --- mechanistic insights --- ligand-exchange reaction --- bridging silylene ligand --- dye-sensitized solar cell --- silylene --- computation --- functionalization --- silicon --- digermene --- N-Heterocyclic tetrylene --- density functional theory --- primary silane --- small molecule activation --- excited state aromaticity --- germanethione --- supramolecular chemistry

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Homogeneous catalysis owes its success, in large part, to the development of a wide range of ligands with well-defined electronic and steric properties, which have thus made it possible to adjust the behavior of many organometallic complexes. However, ligands used in catalysis have long been centered on elements of group 15, and it is only more recently that carbon ligands have proved to be valuable alternatives with the emergence of cyclic diaminocarbenes (NHC).This Special Issue aims to provide a contemporary overview of the advances in carbon ligand chemistry from fundamental aspects to applications.

carbenes --- ylides --- DFT calculations --- electronic structure --- catalysis --- ligands --- structure–activity relationship --- NHC --- nanoparticle --- calixarene --- palladium catalyst --- Suzuki-Miyaura reaction --- amino-acids --- water --- carbon ligand --- amide --- negative charge --- phosphonium ylide --- oxide --- pincer --- metathesis --- ruthenium --- nitro catalysts --- NHC ligands --- olefins --- selenonium ylides --- selenonium salts --- chirality --- stereogenic selenium atom --- asymmetric synthesis --- optical resolution --- reactivity --- malaria --- Plasmodium falciparum --- gold --- NHC-ligands --- hybrid molecules --- drug resistance --- N-heterocyclic carbene --- platinum --- metal complexes --- 195Pt NMR --- N-heterocyclic carbenes --- imidazole --- spectroscopy --- X-ray --- mercury(II) complex --- T-shaped --- carbodiphosphorane --- phosphorus ylides --- pincer ligands --- coordination chemistry --- Cu(I) complex --- photoluminescence --- titanium --- hafnium --- copolymerization of epoxide with CO2 --- density functional theory --- natural bond orbitals --- aromaticity --- ion pairs --- alkali metals --- tropylidenyl ions --- cyclooctatetraene ions --- rhodium --- electron paramagnetic resonance (EPR) spectroscopy --- density functional theory (DFT) --- electrochemistry --- carbone complexes --- carbido complexes --- transition metal complexes --- chemical bonding --- pincer ligand --- macrocycle --- lithium --- potassium --- intramolecular C-H activation --- dehydrogenation --- carbone --- ligand --- germylene --- coordination --- ylide --- n/a --- structure-activity relationship