Resonance structures metal carbene

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

In contrast to the above ruthenium(O) complexes, the reaction of the Ru(II) complex 13, bearing a cyclopentadienyl (Gp) ligand, with electronically neutral phenyl-acetylene gave rise to the coordinatively unsaturated ruthenacyde 14 (Scheme 4.4) [20]. On the basis of X-ray structural analysis, the original authors claimed that 14 is the first formally 18-electron rathenmm(ll)-metallacYclopentatrwne. In accordance with this claim, the NMR resonance corresponding to the metal-carbene a car-... 

Several approaches exist to understanding the structure of metal carbenes. Perhaps the simplest and most familiar is resonance theory. Structures 10-13 represent several possible contributing resonance structures of metal carbenes. Structures 10-12 seem to be important contributors for Fischer carbene complexes, as indicated by experiment and calculations. 

Because of this compensation effect the difference in the p f values of entries 5 and 6 remains small. The fact that the p f difference between entries 11 and 12 (Ap f = 0.68) is larger than that between entries 5 and 6 (Ap f = 0.14) supports this explanation the resonance structure 149 of entries 11 and 12 is of lesser importance than 148 and hence there is less negative charge on the (CO)5M moieties of entries 11 and 12 than those of 5 and 6. As a result there is less compensation and less reduction in Ap f between the chromium and tungsten carbene complex. The same type of compensating effects may be involved in explaining the small metal dependence of the equilibrium constants for nucleophilic addition to the carbene carbon (Tables 8, 9, 14, 17 and 19). 

Carbenes are defined as species containing divalent carbon [1], and they may display either electrophilic or nucleophilic reactivity depending on whether the two unshared electrons on the carbon center are unpaired (triplet carbene) or paired (singlet carbene). Metal-carbene complexes can be classified in a similar way based on their reactivity toward electrophiles and nucleophiles. The resonance forms shown in Fig. 4.1 define the limiting structures, and the formal charge on the carbene carbon indicates the preferred reactivity. Those that are nucleophilic at carbon are called Schrock-type complexes or alkylidenes, and they generally... 

At the other extreme, metal carbenes that are electrophilic at carbon are called Fischer-type complexes, and they generally contain jt-donating heteroatom substituents [4], Fischer reported the first example in 1964 [5], In these cases, the metal-carbene interaction can be represented by three resonance structures, the first with a formal M=C double bond, the second with a M-C single bond and charge separation, and the third with additional multiple bond character between the carhon and the heteroatom substituent. 

Nuclear magnetic resonance (nmr) studies have indicated the partial doublebond character of the carbon-heteroatom bond in the metal-carbene complexes as depicted in resonance structure Ic. The barrier to rotation about the carbon-nitrogen bond in II was found to be greater than 25 Real mole ... 

Our earliest studies concerned the mechanism by which thiamine pyrophosphate (1) acts as a coenzyme (Fig. 1.1). We discovered that the C-2 hydrogen on the thiazolium ring of thiamine was able to ionize to form a thiazolium ylide (2) that has the important resonance structure (3), which can be called a stabilized carbene . We pointed out this carbene contribution to its structure, related to the well-known hybrid structure of carbon monoxide, and also saw that an imidazolium ion (4) and an oxazoKum ion (5) could form such yhde/carbene resonance structures. Recently such structures, usually referred to as stabilized carbenes , have proven to be useful ligands for catalytic metal ions. The carbenes are of course stabilized by electron donation from the heteroatoms, forming the ylide resonance forms. 

Figure 1. Resonance structures for NHCs The stebility of the metal complexes used as therapeutic radiopharmaceuticals is extremely in ortant. A major problem in diis area is dissociation of the metal from die metal conqilex that is administered to the patient. This can result in the accumulation of radioactive elements in healthy tissue of the patient. N-heterocyclic carbenes bind very strongly to transition metals and current data indicate that N-heterocyclic carbenes will bind more strongly to metal centers than do the other chelating and monodentate ligands used in the above applications. The metals to be focused on will be silver and rhodium because " Ag and Rh show promise for use in radiation therapy as a 0 particle emitting radionuclides. Both " Rh and " Ag have convenient half-lives of 1.5 days and 7.47 days, respectively. Both of these radioisotopes emit relatively low levels of imageable y-radiation, with significantly less y-radiation than l which is used in the treatn nt of thyroid cancer. The high level of y-radiation of has been considered one of its drawbacks.

The term metalla-/3-diketonate refers to an anionic diacyl complex, such as L M(RCO)(R CO) , in which the two acyl ligands occupy adjacent coordination sites of the metal atom. Resonance stabilization of the negative charge by 7r-delocalization onto the two acyl ligands gives Lewis structures 1 and 2. The two acyl ligands acquire an acyl/carbene... 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

When the metal is niobium, the two metal frontier orbitals, dy and dz2, are closer in energy and match the corresponding orbitals of triplet free carbene. Figure 10-4b provides a picture of the interactions involved (only the important orbital interactions are shown). The elections in the M-C bond are much more equally distributed between the metal and Ccarbene (because the energy levels of the metal and carbon orbitals are comparable) than with an electrophilic carbene complex. In resonance terminology, this means that structure 10 becomes an important contributor to the overall structure of this particular Schrock carbene complex.11... 

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