In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms were fully ionic. It describes the degree of oxidation of an atom in a chemical compound. Conceptually, the oxidation state may be positive, negative or zero. While fully ionic bonds are not found in nature, many bonds exhibit strong ionicity, making oxidation state a useful predictor of charge.
In chemistry, aromaticity means a molecule has a cyclic (ring-shaped) structure with pi bonds in resonance. Aromatic rings give increased stability compared to saturated compounds having single bonds, and other geometric or connective non-cyclic arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability. The term aromaticity with this meaning is historically related to the concept of having an aroma, but is a distinct property from that meaning.
A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.
Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.
Cyclodecapentaene or [10]annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry.
A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.
Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective insecticides, although some are extremely toxic to humans, including sarin and VX nerve agents.
Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example, fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.
Diphosphorus is an inorganic chemical with the chemical formula P
2. Unlike nitrogen, its lighter pnictogen neighbor which forms a stable N2 molecule with a nitrogen to nitrogen triple bond, phosphorus prefers a tetrahedral form P4 because P-P pi-bonds are high in energy. Diphosphorus is, therefore, very reactive with a bond-dissociation energy (117 kcal/mol or 490 kJ/mol) half that of dinitrogen. The bond distance has been measured at 1.8934 Å.
Hexazine is a hypothetical allotrope of nitrogen composed of 6 nitrogen atoms arranged in a ring-like structure analogous to that of benzene. As a neutrally charged species, it would be the final member of the azabenzene (azine) series, in which all of the methine groups of the benzene molecule have been replaced with nitrogen atoms. The two last members of this series, hexazine and pentazine, have not been observed, although all other members of the azine series have.
Boron monofluoride or fluoroborylene is a chemical compound with formula BF, one atom of boron and one of fluorine. It was discovered as an unstable gas and only in 2009 found to be a stable ligand combining with transition metals, in the same way as carbon monoxide. It is a subhalide, containing fewer than the normal number of fluorine atoms, compared with boron trifluoride. It can also be called a borylene, as it contains boron with two unshared electrons. BF is isoelectronic with carbon monoxide and dinitrogen; each molecule has 14 electrons.
Diborane(2), also known as diborene, is an inorganic compound with the formula B2H2. The number 2 in diborane(2) indicates the number of hydrogen atoms bonded to the boron complex. There are other forms of diborane with different numbers of hydrogen atoms, including diborane(4) and diborane(6).
A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
Silylones are a class of zero-valent monatomic silicon complexes, characterized as having two lone pairs and two donor-acceptor ligand interactions stabilizing a silicon(0) center. Synthesis of silylones generally involves the use of sterically bulky carbenes to stabilize highly reactive Si(0) centers. For this reason, silylones are sometimes referred to siladicarbenes. To date, silylones have been synthesized with cyclic alkyl amino carbenes (cAAC) and bidentate N-heterocyclic carbenes (bis-NHC). They are capable of reactions with a variety of substrates, including chalcogens and carbon dioxide.
The triboracyclopropenyl fragment is a cyclic structural motif in boron chemistry, named for its geometric similarity to cyclopropene. In contrast to nonplanar borane clusters that exhibit higher coordination numbers at boron (e.g., through 3-center 2-electron bonds to bridging hydrides or cations), triboracyclopropenyl-type structures are rings of three boron atoms where substituents at each boron are also coplanar to the ring. Triboracyclopropenyl-containing compounds are extreme cases of inorganic aromaticity. They are the lightest and smallest cyclic structures known to display the bonding and magnetic properties that originate from fully delocalized electrons in orbitals of σ and π symmetry. Although three-membered rings of boron are frequently so highly strained as to be experimentally inaccessible, academic interest in their distinctive aromaticity and possible role as intermediates of borane pyrolysis motivated extensive computational studies by theoretical chemists. Beginning in the late 1980s with mass spectrometry work by Anderson et al. on all-boron clusters, experimental studies of triboracyclopropenyls were for decades exclusively limited to gas-phase investigations of the simplest rings (ions of B3). However, more recent work has stabilized the triboracyclopropenyl moiety via coordination to donor ligands or transition metals, dramatically expanding the scope of its chemistry.
Alexander I. Boldyrev was a Russian-American computational chemist and R. Gaurth Hansen Professor at Utah State University. Professor Boldyrev is known for his pioneering works on superhalogens, superalkalis, tetracoordinated planar carbon, inorganic double helix, boron and aluminum clusters, and chemical bonding theory, especially aromaticity/antiaromaticity in all-metal structures, and development of the Adaptive Natural Density Partitioning (AdNDP) method.
Borepins are a class of boron-containing heterocycles used in main group chemistry. They consist of a seven-membered unsaturated ring with a tricoordinate boron in it. Simple borepins are analogues of cycloheptatriene, which is a seven-membered ring containing three carbon-carbon double bonds, each of which contributes 2π electrons for a total of 6π electrons. Unlike other seven-membered systems such as silepins and phosphepins, boron has a vacant p-orbital that can interact with the π and π* orbitals of the cycloheptatriene. This leads to an isoelectronic state akin to that of the tropylium cation, aromatizing the borepin while also allowing it to act as a Lewis acid. The aromaticity of borepin is relatively weak compared to traditional aromatics such as benzene or even cycloheptatriene, which has led to the synthesis of many fused, π-conjugated borepin systems over the years. Simple and complex borepins have been extensively studied more recently due to their high fluorescence and potential applications in technologies like organic light-emitting diodes (OLEDs) and photovoltaic cells.
Organoberyllium chemistry involves the synthesis and properties of organometallic compounds featuring the group 2 alkaline earth metal beryllium (Be). The area remains understudied, relative to the chemistry of other main-group elements, because although metallic beryllium is relatively unreactive, its dust causes berylliosis and compounds are toxic. Organoberyllium compounds are typically prepared by transmetallation or alkylation of beryllium chloride.
Boraacenes are polycyclic aromatic hydrocarbons containing at least one boron atom. Structurally, they are related to acenes, linearly fused benzene rings. However, the boron atom is electron deficient and may act as a Lewis Acid when compared to carbon. This results in slightly less negative charge within the ring, smaller HOMO-LUMO gaps, as well as differences in redox chemistry when compared to their acene analogues. When incorporated into acenes, Boron maintains the planarity and aromaticity of carbon acenes, while adding an empty p-orbital, which can be utilized for the fine tuning of organic semiconductor band gaps. Due to this empty p orbital, however, it is also highly reactive when exposed to nucleophiles like water or normal atmosphere, as it will readily be attacked by oxygen, which must be addressed to maintain its stability.
Robert J. Gilliard, Jr. is an American chemist and researcher who is the Novartis Associate Professor of Chemistry at Massachusetts Institute of Technology. His research involves the synthesis of molecules for energy storage, molecular materials, and main-group element mediated bond activation. He is a member of the editorial advisory board at Inorganic Chemistry, Chemical Communications, and Angewandte Chemie, among other scientific journals.
