Fischer-type carbenes electronic structure

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

An aminophosphinidene complex Cp Os(CO)2 PCl(N-Tr2) was prepared by reaction of [Cp Os(CO)2] with Tr2NPCl2. As with Fischer carbenes (see Fischer-type Carbene Complexes), the nitrogen group stabilizes the electron-deficient phosphorus atom of the phosphinidene ligand. Based on a crystal structure of the ruthenium analog, there is likely... 

There are several parallels between the chemistries of the carbene and carbyne ligands. The classification of carbyne complexes into two major structural types—Fischer and Schrock—is perhaps the most obvious of these parallels. Complex 64 represents the prototypical Fischer carbyne complex Ccarbyne bonded to a low-valent metal with Ji-accepting CO ligands attached. Structure 65, on the other hand, is a classic example of a Schrock carbyne complex because a high-valent metal is present with electron-donating ligands attached. Atoms attached to Ccaibyne helpful in distinguishing between Fischer and Schrock carbene complexes (i.e., heteroatoms for the former and H and C for the latter), are less important in the case of carbyne complexes. It is convenient to classify carbyne complexes... 

The electron-deficient Fischer carbene carbon is affected by the presence of the lone pair(s) of its rr-donor substituents, denoted ORflp). Structure 11.1 shows how the ) and OR(lp) orbitals compete fw n bondii to the carbene carbon. This can be described in VB (valence bond) language by resonance between 11.2 and 11.3. The real structure often re.sembles lU rather than 11.2 as shown by tiie long M-C and short C-O bonds found by X-ray studies. For electron counting purposes we regard the Fischer carbene as an L-type ligand such as CO. Note that the true M=C bond order is less than 2, thanks to die contribution of 11.3. 

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