In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R-(C:)-R' or R=C: where the R represent substituents or hydrogen atoms.
π backbonding, also called π backdonation, is a concept from chemistry in which electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a π-acceptor ligand. It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge. Compounds where π backbonding occurs include Ni(CO)4 and Zeise's salt. IUPAC offers the following definition for backbonding:
A description of the bonding of π-conjugated ligands to a transition metal which involves a synergic process with donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from an nd orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-antibonding orbital of the ligand.
A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes.
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.
The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals of transition metals consist of five d orbitals, one s orbital and three p orbitals which can collectively accommodate 18 electrons as either bonding or nonbonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period. The rule is not helpful for complexes of metals that are not transition metals, and interesting or useful transition metal complexes will violate the rule because of the consequences deviating from the rule bears on reactivity. The rule was first proposed by American chemist Irving Langmuir in 1921.
In organometallic chemistry, the Green–Davies–Mingos rules predict the regiochemistry for nucleophilic addition to 18-electron metal complexes containing multiple unsaturated ligands. The rules were published in 1978 by organometallic chemists Stephen G. Davies, Malcolm Green, and Michael Mingos. They describe how and where unsaturated hydrocarbon generally become more susceptibile to nucleophilic attack upon complexation.
Organoruthenium chemistry is the chemistry of organometallic compounds containing a carbon to ruthenium chemical bond. Several organoruthenium catalysts are of commercial interest and organoruthenium compounds have been considered for cancer therapy. The chemistry has some stoichiometric similarities with organoiron chemistry, as iron is directly above ruthenium in group 8 of the periodic table. The most important reagents for the introduction of ruthenium are ruthenium(III) chloride and triruthenium dodecacarbonyl.
Transition metal carbyne complexes are organometallic compounds with a triple bond between carbon and the transition metal. This triple bond consists of a σ-bond and two π-bonds. The HOMO of the carbyne ligand interacts with the LUMO of the metal to create the σ-bond. The two π-bonds are formed when the two HOMO orbitals of the metal back-donate to the LUMO of the carbyne. They are also called metal alkylidynes—the carbon is a carbyne ligand. Such compounds are useful in organic synthesis of alkynes and nitriles. They have been the focus on much fundamental research.
A metal-phosphine complex is a In coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).
Mesoionic carbenes (MICs) are a type of reactive intermediate that are related to N-heterocyclic carbenes (NHCs) and are used in scientific research in chemistry. Unlike simple NHCs, the canonical resonance structures of these carbenes are mesoionic: an MIC cannot be drawn without adding additional charges to some of the atoms. MICs re also called abnormal N-heterocyclic carbenes (aNHC) or remote N-heterocyclic carbenes (rNHC). A variety of free carbenes can be isolated and are stable at room temperature. Other free carbenes are not stable and are susceptible to intermolecular decomposition pathways. MICs do not dimerize according to Wanzlick equilibrium as do normal NHCs. This results in relaxed steric requirements for mesoionic carbenes as compared to NHCs. There are several mesoionic carbenes that cannot be generated as free compounds, but can be synthesized as a ligand in a transition metal complex. Most MIC transition metal complexes are less sensitive to air and moisture than phosphine or normal NHC complexes. They are also resistant to oxidation. The robust nature of MIC complexes is due to the ligand’s strong σ-donating ability. They are stronger σ-donors than phosphines, as well as normal N-heterocyclic carbenes due to decreased heteroatom stabilization. The strength of carbene ligands is attributed to the electropositive carbon center that forms strong bonds of a covalent nature with the metal. They have been shown to lower the frequency of CO stretching vibrations in metal complexes and exhibit large trans effects.
Diiminopyridines are a class of diimine ligands. They featuring a pyridine nucleus with imine sidearms appended to the 2,6–positions. The three nitrogen centres bind metals in a tridentate fashion, forming pincer complexes. Diiminopyridines are notable as non-innocent ligand that can assume more than one oxidation state. Complexes of DIPs participate in a range of chemical reactions, including ethylene polymerization, hydrosilylation, and hydrogenation.
Carbene radicals are a special class of organometallic carbenes. The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex using diazo compounds and related carbene precursors. Cobalt(III)-carbene radicals have found catalytic applications in cyclopropanation reactions, as well as in a variety of other catalytic radical-type ring-closing reactions.
In chemistry, cyclic(alkyl)(amino)carbenes (CAACs) are a family of stable singlet carbene ligands developed by Prof. Guy Bertrand and his group in 2005 at UC Riverside. In marked contrast with the popular N-heterocyclic carbenes (NHC) which possess two "amino" substituents adjacent to the "carbene" center, CAACs possess one "amino" substituent and an sp3 carbon atom "alkyl". This specific configuration makes the CAACs very good σ-donors and π-acceptors when compared to NHCs. Moreover the reduced heteroatom stabilization of the carbene center in CAACs versus NHCs also gives rise to a smaller ΔEST.
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.
Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates.
An N-Heterocyclic Silylene (NHSi) is an uncharged heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.
Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Because isocyanide are relatively basic, but also good pi-acceptors, a wide range of complexes are known. Some isocyanide complexes are used in medical imaging.
A Fischer carbene is a divalent organic ligand in an organometallic compound. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-back donation from the metal centre is generally weak, the carbene carbon is electrophilic.
Coinage metal N-heterocyclic carbene (NHC) complexes refer to transition metal complexes incorporating at least one coinage metal center (M = Cu, Ag, Au) ligated by at least one NHC-type persistent carbene. A variety of such complexes have been synthesized through deprotonation of the appropriate imidazolium precursor and metalation by the appropriate metal source, producing MI, MII, or MIII NHC complexes. While the general form can be represented as (R2N)2C:–M (R = various alkyl or aryl groups), the exact nature of the bond between NHC and M has been investigated extensively through computational modeling and experimental probes. These results indicate that the M-NHC bond consists mostly of electrostatic attractive interactions, with some covalent bond character arising from NHC to M σ donation and minor M to NHC π back-donation. Coinage metal NHC complexes show effective activity as catalysts for various organic transformations functionalizing C-H and C-C bonds, and as antimicrobial and anticancer agents in medicinal chemistry.
