Carbene analogs in chemistry are

carbenes In organic 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 or where the R represents substituents or hydrogen atoms. The term "carbene" may ...

with the

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

atom replaced by another

chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

. Just as regular carbenes they appear in

chemical reactions A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of

group 13 The Group 13 network ( pl, Trzynastka, Yiddish: ''דאָס דרײַצענטל'') was a Jewish Nazi collaborationist organization in the Warsaw Ghetto during the German occupation of Poland in World War II. The rise and fall of the Gro ...

group 14 The carbon group is a periodic table group consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and flerovium (Fl). It lies within the p-block. In modern IUPAC notation, it is called group 14. In the field of semico ...

, group 15 and group 16.

Group 13 carbene analogs

In group 13 elements the boron carbene analog is called a borylene or boranylidene.

Group 14 carbene analogs

The heavier

carbenes are

silylene Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exis ...

s, R₂Si:, germylenes R₂Ge: (example

diphosphagermylene Diphosphagermylenes are a class of compounds containing a divalent germanium atom bound to two phosphorus atoms. While these compounds resemble diamidocarbenes, such as Persistent carbene, N-heterocyclic carbenes (NHC), diphosphagermylenes displa ...

stannylenes Stannylenes (R2Sn:) are a class of organotin chemistry, organotin(II) compounds that are analogues of carbene. Unlike carbene, which usually has a triplet ground state, stannylenes have a singlet ground state since valence orbitals of tin (Sn) hav ...

R₂Sn: and plumbylenes R₂Pb:, collectively known as metallylenes and regarded as

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

s for polymetallanes. The

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

for these compounds is +2 and stability increases with

principal quantum number In quantum mechanics, the principal quantum number (symbolized ''n'') is one of four quantum numbers assigned to each electron in an atom to describe that electron's state. Its values are natural numbers (from 1) making it a discrete variable. ...

(moving down a row in the

periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ...

). This makes dichloroplumbylene PbCl₂ and dichlorostannylene SnCl₂ stable

ionic compound In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ...

s although they exist as polymers or ion pairs. Group 14 carbene analogs do not form hybrid orbitals but instead retain (ns)²(np)² electron configuration due to the increasing s p gap for larger elements. Two electrons remain in an s-orbital and therefore their compounds have exclusively singlet ground states and not the triplet ground state which can be observed in carbenes depending on the substituents. The s-orbital (

lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC '' Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. L ...

) is inert and the vacant

p-orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...

is very reactive. Stable group 14 carbenes require stabilization of this p-orbital which is usually accomplished by coordination of a Cp* ligand or coordination to

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

containing ligands, although stabilization can be achieved through steric protection alone. General methods for the synthesis of carbon-substituted (

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...

) metallylenes are reduction of M⁴⁺ species or

substitution reaction A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...

s at M²⁺ halides. Stable metallylenes require bulky substituents in order to prevent nucleophilic attack of the metal center at the p-orbital. Examples of these bulky substituents in R₂M: are

mesityl Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenze ...

, Dis (di(trimethylsilyl)methyl) and adamantyl groups. With insufficient steric shielding the metallylene will form a dimer or a

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

. The first isolable dialkylgermylene was synthesised in 1991: :Me₅C₅GeCl + LiCH(Si(Me₃))₂ → Me₅C₅GeCH(Si(Me₃))₂ :Me₅C₅GeCH(Si(Me₃))₂ + LiC(Si(Me₃))₃ → (SiMe₃)₃CGeCH(Si(Me₃))₂ Stable diarylgermylenes also require bulky ligands: :Ge (SiMe₃)₂sub>2 + 2 LiC₅H₃(C₁₀H₇)₂ → Ge iC₅H₃(C₁₀H₇)₂sub>2 The C-M-C

bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemi ...

in metallylenes is less than 120° confirming hybridization other than sp². The higher p-character for the C-M^II bond compared to the C-M^IV bond is reflected in its slightly higher

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest ...

N-heterocyclic silylene 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 compoun ...

s are known to be stable for months and have been studied extensively.

Group 15 carbene analogs

In the

group 15 elements A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the el ...

the neutral

carbene analog (RN) is called a

nitrene In chemistry, a nitrene or imene () is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and univalent, so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded electrons. It is therefore cons ...

. The

analog is a phosphinidene. There are charged group 15 carbene analogs as well, most notably phosphenium ions (R₂P⁺) which are isolobal with (hetero-)carbenes possessing a singlet ground state.

Group 16 carbene analogs

Carbene analogs of

group 16 element The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...

s have been first reported in 2009.''Reactive Dications Tamed Main-Group Chemistry: Stabilizing positive charge with three nitrogen atoms yields stable group 16 complexes'' Steve Ritter

Chemical & Engineering News ''Chemical & Engineering News'' (''C&EN'') is a weekly news magazine published by the American Chemical Society, providing professional and technical news and analysis in the fields of chemistry and chemical engineering.Link
/ref>

Sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and telluriu ...

and

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fo ...

dication A dication is any cation, of general formula X2+, formed by the removal of two electrons from a neutral species. Diatomic dications corresponding to stable neutral species (e.g. formed by removal of two electrons from H2) often decay quickly into ...

s have been found to be stabilized by the diiminopyridine ligand DIMPY. For example, the reaction product of

triflate In organic chemistry, triflate ( systematic name: trifluoromethanesulfonate), is a functional group with the formula and structure . The triflate group is often represented by , as opposed to −Tf, which is the triflyl group, . For example, ...

S(Otf)₂ and (2,6-diisopropylphenyl)₂DIMPY at -78 °C results in an air-stable dicationic sulfur compound with a naked S²⁺ atom coordinated by three nitrogen atoms by

dative bond In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal i ...

s. :

References

{{Reflist Carbenes