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Overview
C=M complexes is a band that containing metal-carbon double bonds. This double bond consist of a σ bond and a π bonds. There are two main carbene complexes known as Fischer type(carbene) or Schrock type(alkylidene). The first synthesis was developed by Fisher in 1964. This first synthesis are called as Fisher-type carbene complexes that containing a σ donor on carbon and a π-back donate on metal. After several years Fischer-type carbene complexes are developed, these have studied more extensively and more broadly by Schrock. The second synthesis are called as Schrock-type carbene complexes, containing more over sharing electrons and commonly known as alkylidenes. Yves Chauvin, Robert Grubbs, Ricahrd Schrock were awarded the 2005 Nobel prize in chemistry. "for the development of the metathesis method in organic synthesis."

Fisher-Type Carbene Complexes
A carbon on Fisher type is electrophilic because σ donate from the metal to the carbon and has weak back bonding. Carbon complexes on Fisher has low oxidation state with 18 electron count. For example, Fe(0), Mo(0), Cr(0) (middle to late transition metal) containing good π acceptors ligands in the complex. It contains highly electronegative heteroatoms such as O,N, or S. Heteroatoms directly attached to the carbene, and make more stable Fischer-Type carbene complexes. These characteristic of Fischer-Type carbene complex allows to have isomerization double bond with Oxygen. The distance of these double bond in complexes length are longer than normal M=C double bond, and shorter than normal M-C single bond. Likewise, the C-X bond distance is shorter than normal C-X bond. The highly electronegative heteroatoms like to participate in the π-back bonding with d orbital on the metal, and p orbitals on the carbon. A lone pair is donated from a carbon to an empty d orbital on the metal. A lone pair is donate from π back donate metal to $$P_z$$ orbital on carbon. An electron pair corporate in σ donation and there are empty π* orbital which takes π back-donation in MO diagram. Bonding is most likely close to CO bond, and good electrons for back donation with good π acceptor. An example of Fischer-Type carbene complexes is the compound $$Cr(CO)_5[C(OCH_3)C_6H_5]$$ with a Cr(0).

These resonance form will existing on a temperatrue-dependent proton NMR (detects the cis and trans separately), and on a carbon NMR. however, π bonding system in complexes of this type will highly more existing on X-ray crystallography(shows double bond character).

Schrock-Type Carbene Complexes
A carbon on Schrock-Type is nucleophilic, because backbonding metal is strong and does contribute for σ donation from the ligand and the metal. Carbon complexes on Schrock has high valence oxidation state with 10-18 electron count. For example, Ti(IV), Ta(V), W(VI) (early transition metal) containing good σ or π donor ligands in the complex. An electron help to form σ bond and π bond in MO diagram. It mostly like compose with H or alkyl. H or alkyl directly attached to the carbene. These complexes is composed with two covalent bond interactions. one electro donates to the σ bond from each metal and each carbene. An example of Schrock-Type carbene complexes is the compound $$Cp_2(Me)Ta=CH_2 $$ with a Ta(V).

These Schrock-Type carbene complexes have M-C-R linkage and make bond angle of 160-170°. Schrock Carbenes can be display nucleophilic carbene complexed by proton NMR, carbon NMR, Infrared Spectroscopy, and Raman Spectroscopy. These complexes display coupling constant values for typical and agostic interaction between the carbene proton and the metal. These techniques are well used to determine bond angles and structures for Schrock-Type carbene complexes.

Fisher Carbenes
Fisher Carbenes' carbon is an electrophilic. Nucleophilic attack at a carbonyl ligand, and most common method.

Alkyl lithium is attaching on metal carbonyl. Zwitterionic resonance is forming by attaching heteroatom to carbene to stabilize Fischer carbenes. Intermediate of Fisher Carbenes is treated as electrophilic to give the Fisher carbene. C-X bond rotation is restricted in syn and anti isomers for alkyl derivatives at low temperature proton NMR because of zwitterion resonance form in Fischer carbenes. These can be observed by X-ray crystallography.

Schrock Carbenes
Schrock Carbenes' carbon is a nucleophilic. There is α-abstraction in the Schrock carbenes synthesis and induced by steric bulk. Schrock carbene is the main key for the both reagents and catalysts. The most famous examples for Schrock Carbenes synthesis are Patasis' Reagent, Tebbe's Reagent, and Grubb's Catalyst. Schrock Carbenes synthesis is used to widespread reactivity such as intermediate of the preparation of organometallic. The most famous of Schrock Carbenes reactivity is Olefin Metathesis.

Olefin Metathesis
Olefin metathesis is the main application for carbene. Olefin metathesis reaction is using two alkenes to make cyclobutanes and reform the two new types of double bond. A reaction of olefin metathesis work rapidly. D-orbitals on the metal alkylidene is presence, and it breaks cyclobutanes symmetry and reacts very quickly. Normally, the products of olefin metathesis are statistical, unless the reaction can be driven is some way or the tow alkenes have different reactivities. Titanium, Tungsten, Molybdenum, and Ruthenium are the popular metal for olefin metathesis.

There are five different kinds of reactions process of Olefin Metathesis:
 * Cross metathesis CM
 * ring-closing metathesis RCM
 * ring-opening metathesis ROM
 * ring-opening metathesis polymerization ROMP


 * acyclic diene metathesis polymerization ADMET