Ligand electron-withdrawing

C-NMR chemical shifts are a useful diagnostic tool for carbene complexes, with Ccarb being substantially deshielded. The Ccarb resonance is seen to shift to higher fields as the electron deficiency of the metal center is increased (i.e., with electron-withdrawing ligands and in cationic complexes). 

In the absence of electron-withdrawing ligands, disilanes possess only a little Lewis acidity consequently, they show a limited propensity to form hypervalent complexes. Starting from the symmetric methylchlorodisilanes ClMe2SiSiMe2Cl and Cl2MeSiSiMeCl2 as well as carboxylic acids and thiocarboxylic acids, Kawashima et al. have prepared rare examples of penta- and hexacoordinated disilene, namely compounds 886-889 (Scheme 125).855 856... 

The following observations are intriguing from both mechanistic and synthetic viewpoints (Scheme 16.92). In the stoichiometric reactions, electron-withdrawing ligands (e.g. dba) on palladium are necessary for the olefmic double bond to attack a jt-allylpalladium group. The resulting electrophilic Jt-allylpalladium group becomes... 

Phosphite ligands (Fig. 9) are known to be better 7r-acceptors than phos-phane ligands. The stronger electron-withdrawing ligand increases the reaction rate because CO from the metal center is dissociated faster than with phosphane ligands. For different mono-phosphites, the selectivity to the... 

It is apparent from the results with the cyclization of 61 to 62 versus 63 (Eq. 4) that more electron-withdrawing ligands on the rhodium make the derived carbene more reactive. These results suggest that more electron-donating phosphines would be the optimum ligands for the preparation of analogs of (M)-60 and (M)-56. Unfortunately, all attempts to prepare such derivatives have failed at the ortho-metallation stage. 

As described in Section II.A.3, dx-bis(4,4 -dicarboxy-2,2 -bipyridine)dithiocya-nato ruthenium(II) (N3 dye) and trithiocyanato 4,4 4"-tricarboxy-2,2 6, 2"-terpyr-idine ruthenium(II) (black dye) exhibit quite good performance therefore, the systems using these two Ru dyes have been intensively investigated. However, other Ru dye photosensitizers have also been synthesized and characterized, and their performances as photosensitizers in DSSCs have been reported by many researchers [98-116]. Several kinds of electron withdrawing ligand for Ru dye photosensitizers are shown in Fig. 13. Figure 14 shows the structures of newly synthesized Ru dye photosensitizers and their absorption properties. The axis shows the molar absorption coefficient, e (i.e., absorption coefficient per M unit M 1 cm-1). 

The complexes with electron withdrawing ligands (L ) on the other hand can decompose water spontaneously... 

Pd(H) complexes with strongly electron-withdrawing ligands can insert into the allylic C—H bond (path c) to form directly the Jt-allyl complex via oxidative addi-tion.502,694,697 Pd(OOCCF3)2 in acetic acid, for example, ensures high yields of allylic acetoxylated products.698 The delicate balance between allylic and vinylic acetoxylation was observed to depend on substrate structure, too. For simple terminal alkenes the latter process seems to be the predominant pathway.571... 

A considerable improvement in the efficiency of the reaction of alkyl diazoacetates with benzenoid systems occurred with the development of rhodium(ll) carboxylates as catalysts.166 As can be seen in the reaction with benzene, rhodium(il) salts with electron-withdrawing ligands were far superior (Scheme... 

Cycloaddition of CS2 to Fe(C=CMeXdppe)(i/-C5H5) affords the a-2H-thiete-2-thione complex (12), which reacts with iodomethane to give 13, which contains perhaps the most strongly electron-withdrawing ligand known (29) ... 

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