Advanced Inorganic Chemistry/C=M Complexes

C=M complex is an organometallic complex containing metal-carbon double bonds, in which a σ bond and a π bond present. There are two main carbene complex types: Fischer type(carbene) and Schrock type(alkylidene). Fischer type carbene complex, which was developed in 1964, contains a σ donor on carbon and a π-back donate on metal. Schrock-type carbene complexes, which was developed by Schrock several years later, contains 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 donates from the metal to the carbon and has weak back-bonding. Carbon complexes on Fisher have low oxidation state with 18 electron count. For example, Fe(0), Mo(0), Cr(0) (middle to late transition metal) contain good π acceptors ligands in the complex. The stability of Fischer-Type carbene complex can also be enhanced by containing highly electronegative heteroatoms such as O, N, and S. These characteristics of Fischer-Type carbene complex allows to have double bond with Oxygen. The length between these two double bonds in complexes is 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 tend 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 and another lone pair is donated from π back donate metal to $$P_z$$ orbital on carbon. An electron pair corporate in σ donation and there is 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 temperature-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 back-bonding metal is strong and contributes σ donation from the ligand to the metal. Carbon complexes on Schrock have 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, which are directly attached to the carbene. These complexes is composed with two covalent bond interactions. one electron 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 displayed by proton NMR, carbon NMR, Infrared Spectroscopy, and Raman Spectroscopy. This complex type displays 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 Carbene
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 zwitterion resonance forms 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 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 utilizes two alkenes to make cyclobutanes and reform the two new types of double bond. A reaction of olefin metathesis works rapidly. d orbitals on the metal alkylidene is present, 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